[ano]

First. As db says, all your insane troops cost 1 gold, so you pay 1 upkeep for 15 of them. And of course, void beings such as lesser othernesses cost nothing.
So the only units that are worth getting rid of are tritons and mad tritons. Others can find their niche. Killing tritons is quite easy - just gateway them to the land province.
As for troops management. You get 3 types of cultist in land provinces. The best ones are hybrid cultists who are amphibious and have 10-40 leadership and a rather big chance of astral pick. They are your main commanders to manage magic freespawns. And also you should recruit Illithid lords when you need both types of leadership or use purple Starpawns