is it theoretically possible to have like 20 stars and 500 planets? In other words, no limitations whatsoever? If so, it'll be truly epic!
It's possible to create and play on MASSIVE masp, but the game wasn't designed or balanced for it. Once your empire gets big enough, you'll have so much money that you literally won't be able to spend it faster than you can earn it.
This ultimately isn't nearly as epic as it sounds, since the AI is actually pretty weak in the late game. Especially if you're using the entrenchment expansion pack, it's just
time consuming to finish off the AI in the late game.
As for capital ship choices, carrier or colony. The colony-class capital ships (Akkan, Progenitor, Jerrasul) will speed up your colonization and give you a powerful economic advantage in the early-game. The carrier-class capital ships are simply the best combat caps for early fights. You
might be able to get away with a Kortul or maybe a Marza or Vulkoras, but those are more niche choices and you need to
really know what you're doing.
In singleplayer you can get away with pretty much anything, since the AI picks its capital ships randomly and doesn't use colony-capital ships effectively even if it does pick them. You have to be careful if it got a carrier-class capital ship and you didn't, though, as it will be very hard to fight that AI in the early-game because of the huge advantage having a carrier-cap gives you.