How much essence is required to summon each slime girl? And where/how do you summon them? In addition, where do you find red and green slime girls (besides for the tower)?
The summoning system was taken out of the game. You can get the recipe for Soul Lanterns at the Whisperwind farm in Begus now. They work similar to pokeballs, as that you can use them in a battle to capture the enemy monstergirl. (Doesn't work with all enemies yet).
There are several places for those slimegirls. For example, you can find Red slimegirls in the South dungeon, and Green slimegirls south at the Bitterroot farm.