Space Empires has always allowed:
- multiple techs in 1 turn (as many as you can finish)
- multiple units in 1 turn. A unit is a small commodity, not self-mobile, which is carried as cargo on ships. Units include fighters (tactical in space, but cannot warp), satellites and mines (immobile in space), weapon platforms (immobile on planets), and troops (tactical on planets). They cost so little that even a tiny planet with no factories can pump out 2-10 of them per turn. (Standard technique: you schedule a tour of planets to build to the height of a helix, where height represents turns-from-now, and move one cargo transport along that helix to pick them all up in a single pass.)
Also, due to gun-range gaming 
, bigger ships are better, at least up to cruiser tech (frigate, destroyer, light cruiser, cruiser) , and so you generally cease to build the cheaper ships. Then your available ship size tech increases way faster than your build-rate tech, and so your turns-per-ship strictly increases, and never goes down toward 0.5.
So I have no idea if you could actually research Shipyard up to level 10 (in Capt. Kwok's Balance Mod) and then build 2+ empty frigate hulls in 1 turn. Anyhoo, I win the game long before then!
It has not proven to be game-breaking to allow dozens of units per turn, with mid-game stockpiles of thousands of them. (You pay their upkeep!) They basically wipe each other out, so you must drop 100+ troops to conquer a homeworld's 100. They're fodder for any well-prepared ship, e.g. one with gun range 1+ hexes longer than their cannons 
Once the AIs reach shield tech and start spamming weapon platforms with shield generators ... then planetary bombardment becomes very tedious and not fun, and most players quit and restart. So it's a race to kill all 17 AIs before any of them get that far. Disable their tech trading!
I don't know if SE* allows multiple ships in 1 turn (from one planet/shipyard), because ships generally cost more than 1 shipyard facility's per-turn build rate. N.B. SE*'s ship-spam throttle is that your shipyard facility has a build rate in materials/turn, regardless of how many resources you actually produce.
- Standard work-around: Build a shipyard starbase, which is an upkeep boondoggle, but has its own puny shipyard component, which can thereafter build another ship independently, but at a slower rate. No limit to the number of yardbases you can have in orbit. The stock AI is hard-coded to build that first, which I call the Yardbase Gambit. I've worked out the math, and it sucks eggs: not worth the cost, you'll run your yolk dry and have to suspend one or both of your shipyard queues long before your economy gets rich enough to support two queues. Heck, I only ever gain a shipyard by conquering an enemy homeworld, 1 per homeworld, and even I can't pay to run all N of those shipyards at full throttle all of the time.
- Ridiculous/silly exploit: Have your first yardbase build another yardbase!! (Or, a shipyard ship build another of its own type.) Yes, this enables exponential growth, if you can pay for it. (No, you can't pay for it in early game.) But in mid-late game ...
- I did this once, and it had me bouncing up and down in my chair for two hours because it was sooooooo ba-rokenly fun. SE3, mid-game, I had already conquered about 6 AIs, so my empire was filthy rich, with more resources saved than I could spend. My rejuv (unarmed boarder/shipyard ship, to scavenge crippled enemy hulls) warped blindly into an enemy fleet, retreated from tactical combat out of the space warp hex, and thereafter could not return (because that fleet is sitting on the warp hex). So it had to sprint through the system and escape through another warp ... which strands it far from home, right? The neighboring system was ... a space storm system, no star and no planets, just storms. (It's an SE* thing.) Oh noes, it's trapped, and far from resupply! But -- it is a rejuv ...
- SE* game rule: All shipyard build queues (for both planet facility and ship component) draw freely from your empire's stored resources, regardless of map distance or connectivity.
- SE* game rule: Newly-constructed ships have full supply. If they include supply components, or weapons with ordnance, those are constructed fully loaded, too.
- SE* game rule: A ship with shipyard component can build units. However, it must also have empty cargo space, or the built units are lost immediately.
- So ... my rejuv burrowed in, like an ant queen newly shedding its wings after its one-and-only mating flight, and ... laid eggs. It built another rejuv, those two built two more. Four rejuvs then built light cruiser attack ships. My original rejuv then borrowed some supply points from some of its babies, yup yup.
- One of the new rejuvs sacrificed engines for cargo bays. It built ... troops
- Two hours later, my rejuv + fleet of about 20 light cruisers warped back into the enemy system and pacified it: shot down all ships, took down all fighter swarms with their point-defense, bombarded all weapon platforms into rubble, and conquered some small outlier planets (Mars-ish). Those little planets promptly started to build 120 troops for the homeworld invasion, and a troop transport ship to carry them. (IIRC, I quit that game after that night -- there was no topping that trick.)
- So, yah, my one ship, in an empty system with no resources, trapped and disjoint from my empire, with no hope of escape or rescue ... conjured up an invasion fleet out of nothing. And I had enough resources stockpiled to pay for 500+ ships, so I could have repeated this 25 times, except that there did not exist 25 more AIs who needed it.
I believe you can do the same trick in SE4 and SE5, since a shipyard ship has the same unblockable access to your empire's resource treasury, and it can build another of itself. (I never needed to; I won games too fast to ever have that much build time, haha.) More deeply, this larger/broader concept of a bootstrapping invasion (or, generically, threat from situated exponential growth) is a lovely pattern/concept/nugget, which may be worth exploring at game-design time. There are some nifty parallels with biological life:
- Bacterial infection. Bacteria are optimized to do exactly one thing in life: to convert local resources into more copies of themselves ASAP. Let one bacterium find a food supply, and you get billions of bacteria. The analogy leaks because: teleporting empire-wide resources to a ship anywhere on the map, regardless of reachability, violates "locality" of food supply.
- Viral infection. Viruses hijack a cell's existing RNA-transcription machinery to have it produce more copies of the virus, until the cell literally pops like an overfilled water balloon. Hence, a virus does not carry its own machinery, and is dormant without one. When my invasion skirmishers drop onto a tiny, poorly-defended planet-with-shipyard (it's an SE* AI thing), I gain a shipyard for cheap, which is very much like hijacking the enemy's own machinery to build more copies of my troops. The analogy leaks because: I'z not dormant, mu-ha-ha.
This situated exponential growth trick, as done in SE3, does not even violate GC3's restriction of one ship per turn, because the SE3 build rates are completely normal, and they never exceed the threshold. GC3's mechanism of shipyard decay with distance, and the lack of a shipyard component, adequately prevents this -- yay or boo. Stepping back (or up a few tiers), we could re-envision this as, say, a campaign-level threat or endgame pacification primitive:
- Sun-killer torpedo. If you fail to shoot it down, then on turn T+20, your sun goes nova. The end (for you).
- Ant queen. She runs through your home system, and hides under a rock. If you fail to shoot her down, then on turn T+20, she comes back with a fully-loaded invasion fleet.
- You've Got Bad Data. Your own build orders, using your own shipyards, suddenly ... declare sentience, independence, and war upon you. Thereafter, they intelligently re-add themselves to your own queues, change your UI to disable your mouse clicks, replace your bridge components with Rogue AI components, and promptly conquer your own planets.
- He's Got Bad Data. In Covert Intelligence, you compose a treaty to hostile AI, but embed as its text ... a copy of the You've Got Bad Data meme. When AI leader reads your message, he gets bad data.
i.e. the ant queen rejuv ship is the "uber-weapon" that kills off one entire enemy system per shot. Like a very large bullet. It would be hypnotic to watch an RTS-like game at this high level, where your individual shots are like bacteria looking for food, and you judge their success over tens of seconds by watching colored blooms of growth pop into existence and expand like splashes, as portions of the enemy empire on the cloth map just wither and die. Meanwhile, they're bombarding you with the same, and you're racing! Hah! Evolve your defenses to counter that!
Oh ... as for this thread topic, my general solution is unrestricted scripting to automate the micro away however we like, to the limit of the player pools' combined l33tness. If not in this game, then in the next. Think and play at higher levels.
