Servent - I appreciate your suggestions but feel that having to maintain your monsters' happiness/hunger all the time would be tedious. Also, I personally don't consider that mechanic to be worth any improvements it makes to the combat. And because I would like to have lots of monsters, maybe only 3-6 in use at a time, it would be difficult to manage them all.
I like the capture mechanic. I think that growing the crystals in the tower would be an additonal way to gate content. After the player has reached a certain point, they can start growing crystals that are needed to capture monsters and write them into their grimoire. Different monsters may need different crystals based on level or type. Also some crystals might give a higher chance of capture.