Creating a Nomadic Race?

It cannot be done to create anything approaching playable. Without planets, there is no way to have an economy of any kind, or scale anything.

I'm not sure that is ENTIRELY true.  There is a flag in the faction defs that is the following:

<RequiresHomeworld>FOO</RequiresHomeworld>

That can be set to "true" or "false".  

For factions like the Dreadlords, interestingly enough, it is set to "false".

Then there are race traits like "PirateGold1" (which gives 10 credits per turn) and "PirateSmarts1" (which gives 5 research points per turn) and so on.  Might, just might, be ways to play with that if one is crazy creative enuf.  And I think there are ways to play with range as well.

That being said, as happened to RomeoRomantique, anyone who has just copied the Pirate faction and tried to play it has had a crashed game for their troubles.  But that might also be because they are missing almost in their entirety two branches of the tech tree.

It'd be interesting to try to create a planet-less faction in steps.  That is, create a faction via mod as normal but with the <RequiresHomeworld> tag set to false and see what happens.

Then create a faction with a race trait that gives them credits per turn and/or research per turn and see what happens.  Then try the one that gives military production to a shipyard per turn and see what happens.

It's that last one that's probably hairy and where the game will start snarling at you.  In fact, replicating the pirate bases and how they get production might be the most difficult thing of all.  If one  wants to create new ships (and I presume you do), they have to be built SOMEWHERE.

And this is presuming that the reason the game isn't saying "OH NO YOU DIDN'T" is because it presumes everyone important gets a homeworld at the beginning of the game.

If so, the only way out of this I can see is if there is a scenario trigger to destroy/giveaway a planet.  If that is there, then MAYBE it could be run as a scenario that on Turn One the map generates as normal and then the player's homeworld is destroyed/given away, leaving just the starting ships and bases, which are powered by user defined race traits (which theoretically might be pumped up by user defined technologies in their tech trees [after all you'd want them to have a VERY unique tech tree when it comes to research, military production, and economics]).

It's a horrible kludge and one that depends on RequiresHomeworld working for player factions and scenario triggers.  But that's one of the few ways I can see this somewhat often suggested idea working.

Maybe I'll actually try it one month when I'm really bored and I have lots of free time to bang my head against a wall.

Beyond all of that wittering, then there is the basic game play questions:

Are they allowed to create starbases?

How do they win the game? Lore-wise, can they take over other planets?

Do they get to talk to other races and trade with them? If so, how do you stop them from getting colony ships or transports?

How do you eliminate this faction anyway? (SIDE NOTE:  Even after wiping out all pirates on a map or all Dreadlords on a map, I have yet to see a message saying they've been eliminated from the game.  Which is... curious, actually.  Especially for things like the Peacekeepers and Dreadlords since one could wipe them out in GC II).

What actually is their purpose in the game?  Do they just chug along doing their thing?  This is related to the "how do they win"/"how does one destroy them" question, naturally.

Yes, cool factor.  I got that one already.  But what beyond that?  That needs to be answered just as much as "Is it technically possible", IMO, as it would influence at least a dozen or so decisions about their techs and ships and other related things.

Destroy them:

For a migrant fleet, or a race of pirates or a corporation that use outposts, asteroid dwellers etc, its colony ships, or starbases/shipyards, or asteroids would be 'it'. Once they are gone, its gone. It'd be nice if they could move however their territory in the case of the migrant fleet.

In game terms I guess when the production, research and economic production of the race is 0, the game could declare them dead. 

How do they win. - Well I think this idea works strongest when they are alongside the existing empires, but either competing with them directly or existing beside them would work.

Examples of their own victory objectives:

Pirate or Criminal Race - Control, Have an Outpost on 75% of the worlds or systems in the galaxy.
Migrant Fleet - The Rebirth, Have a population equal to at least 75% of the worlds or systems in the galaxy 
Pirate or Criminal - Underworld Kings, have all remaining empires paying you tribute.
Migrant Fleet - Rebuild your race or Destroy your Conquerors, Find and resettle your homeworlds, Reach a dominant military and tech standpoint
Corporate - Economic Dominance, also known as have all the empires of the galaxy in your pocket, paying you tribute.
All - Be the last surviving corporation or criminal enterprise in the galaxy, other races and factions will want to stop you dominating the galaxy in this way.

All of these races or groups follow a theme, so its really designing 1 new race type with small tweaks where it adds to the concept. You could have them all in direct competition with each other as well.

Assuming they don't have their own victory objectives, which would make more sense, and make all of this below not needed:

How does a race that doesn't use planets win with the existing systems, that is an excellent question. Well that would depend on the type of race.

Diplomatic, tech, and ascension could be achieved in the same way, so they are unaffected.

Influence victories wouldn't be possible with this type of race, or perhaps they would and nothing would change. They still can't build standard colony ships, but they can influence colonies in the same way, or the planet joins an 'independent' race, like the pirates are now. I'd just grey influence victory out when this race is in play.

Conquest victories could follow the same rule, or simply remove the colony, or even set it to 'independent' of anyone, the independent union of worlds if you wanted a fancy name. Why have an independent setting, well this allows the pirate or migrant race to keep doing business in the same way while completely removing all empires as per standard gameplay.

This might be doable. 

1) dead homeworld, choice

2) custom starting ships

3) custom components for ships that generate cash

4) yes let them build starbases

5) winning, who cares, it's his mod let the guy roleplay, plus diplomatic winning would be doable

6) don't know how, but ban this race from colony modules.  They can only rule over subjects captured by invasion or culture. 

7) shipyards, need a base production value, since they won't have sponsors, and perhaps a special ship component for their ships to make them cheaper, but less space.  They can then eventually build normal ships and not get a huge advantage when they do acquire some affluent planets. 

Shipyard ability is possible by modding a race trait I believe. 

8) start them with some unique huge hull ship with lots of perks in the beginning that they can't replicate. 

9) diplomatic bonus module for ships needed

Problems? The AI couldn't handle this race, so it's for the player only. The player would need a. Decent tutorial on how to be effective. 

I've tested some of this, a credit producing ship module is easy. #7 is the one that could just screw you.  

How did you create credits with a ship? Just a highly negative maint cost?

Give planets movement points, Change their planet graphic into a really really huge spaceship. Transform their planet maps into starship interior maps.

Profit?

This one's semi-easy, IMO.  Give them a custom name for their first tech in the Colonization branch and DON'T update ShipComponentDefs.xml in the mod.  This way, they never get the tech that unlocks the Colony Module.

And perhaps make the colony ship that is rewarded by anomalies a trade ship or something else.  I believe your Race Mod has custom Awarded ships, right?  Well, if so, just make the Awarded ship that is tied to the Colony one a Trade Ship (with a license) or Something Else Cool.  

Sure, they'd still be able to trade for them.  But that's on the player.