ok heres my idea for gas giants. max population of 0 since the population is entirely in orbit. 0 emergency thing ie. low bombardment defense. i see gas giants as being fleet capacity hubs and tactical hubs. they can use the gases from the planet to fuel more ships and to create munitions. the GG's should also have large gravity wells. on the tech side of things you should make it that the tech for GG's to be colonized can be researched after fire and ice colonization are researched first, they can only do it after the experience of colonizing terrestrial worlds first. this also minimizes GG's impact on fleet size in the beginnig of the game.