Hello, I'm Sanzuwu and I'll just jot down what I know so far about Monster Unity.
Usually you shouldn't do this, you're wasting EXP and stuff. However, certain unities are rather useful (for example: Kodama, Ghost Cat) for all stages of the game. Also, certain monsters cannot be obtained outside of unity. So if you want to [catch them all], you will have to do this eventually.
I'll go down the catalog list and list the unities I know so far, hopefully I'll be adding more as I get farther into the game (and have the EXP to spare):