I like the idea but I think it needs some tweaks, for instance, rather than 15~20 point cost to set it should be a bit cheaper than setting all 3 individually, especially if it gets rid of the normal 3 you are merging which it sounds like is your idea seeing as you are talking about cleaning up the list. The prices I would say should similar to having a 2, 4, and 6 instinct cost only 10, or an 8, 10, and 12 instinct cost 20~25, cheaper than all 3 together but not to cheap either.

Besides that, I think that its a great idea and I would love to see it happen, especially since it would cut down on the slots needed for the stats I want on certain jobs like BLM. The argument that it will be OP or some such doesn't need to come into play here since Monstrosity is OP as it is and hardly anyone does PvP for it, but PvP isn't going to be much harder even with it anyways if you take a real party like you are supposed to.