and dont get me started on a colony beeing able to grow to the size of twice the homeworld in just 20 game turns; that's just plain wrong!
Correct scaling is not one of the best features of this game, unfortunately.
I do think that the initial conditions for the game are very lame. You start your journey to the stars with your homeworld almost empty in terms of infrastructures, and yet you have 3 jump capable starships in orbit (one of which you don't have most of the techs required to build it...). Also being able to build cargo hulls and not large hulls is somewhat weird.
The point of this being, yes, the start is somewhat hasty.
a) make a huge maintenance cost per colony;
Initial Colonies do have a high maintenance, and some projects got theirs increased - some to absurd values? - still there are some projects/ships/bases that do not have maintenance, which is weird.

colony modules should't cary more than 50 (not 500) colonists.
Considering that it's 500 *million* colonists, not 500 - and that's another QA issue in both the Selected Object Context Area and the Ship Loading Window, and for both Colony ships and Transports - and that now you can actually fit those size *20* modules in small and even tiny hulls... I wonder how 500 million people plus starting materials for a colony can fit in, or even be towed by if you want to look at it that way (but then you wouldn't really have a size limit on ships, would you?) in such small hulls, but again, scaling is just plain awful. I don't think they consider these things too much. Better QA would certainly go a long way.
it would be very cool if it was pozzzible to limit the number of colonies a player has at one time by some tech (let's say government system)
Some form of logistics for planets. Why not?