Well, I finally made a mod.
I was frustrated that colonies on T1 and T2 planets couldn't have all building and turret slots filled. The ecosystem seems stable so how come?
So I made a mod that lets you fill your colonies as you would on a T3 planet.
You will still only be able to place 1, 2 or 3 colonies on T1, T2 or T3 planets respectively and T0 planets are unaffected because A/ I don't how to make it affect them and B/ if your ship can be damaged then the other vehicles would be annihilated, so no spice production on T0 colonies.
Hope it will be useful for others than me. Enjoy!