thats a very interesting idea.
Whether or not it is too much for newly gauntleted magi is not so easy to answer. If you are the sole storyguide and you want it in your saga then by default the answer is no, it is not too much. If you feel like trying it out: go for it. The worst that can happen is that it ruins your saga and you have to either end the saga or convince the players to let go of the ring.
If you are using troupe style play you might want to discuss the item with your group as the authority to potentially ruin the saga is not purely yours. One solution could be to give the item to a magus of another person to storyguide frequently and then make that magus a central character in your plots, that way the ringbearing magus can be made to feature only when you storyguide. Do make sure to have an explicit agreement about such an arrangement with the player in question.
I would probably go for either Corpus, Vim, Mentem or Terram in no particular order, but I am sure that you will get posters who favor any different forms.
Regarding the form requisites, I will assume that you chose "wizards boost (animal)" for the sake of the example. If you did then the spell ought to work on any TeAn(Req) spells, that is any spells that are primarily Animal spells. It would however not work on TeFo(An) spells, as those spells are Form spells with an animal requisite. Eq a MuCo(An) spell is a Corpus spell with an Animal requisite, which means that it is not an Animal spell. (the requisite does not count as being the form of the spell, for this purpose).
Be aware of the following: By the standard interpretation of the how hermetic magic works such an item would not work. It is generally considered that items with metamagic powers can only target effects that are enchanted into the item. That should not stop you, and in fact if it were me, I would consider that to be a benefit, because it means that your players cannot easily replicate the item. It might be worth considering why and how the ring came to be though. But since you named your post "Ring of Power?" I am assuming that you are going to make the ring a central point in your saga anyway.