I don't know if you know this. Just ignore if you've known already. Upon reaching V1C6 in Beyond, you'll unlock Specialisations. One of them is Magizoology that we have done since our student days. After you unlock the Magizoology Specialisation, the max level is extended to 20. Afaik, the only known way to get magizoology XP (to level up your Magizoology Rank) is to feed and bond with un-maxed creatures. After you reach LV. 10 Magizoology, any XP you earn from feeding and bonding with un-maxed creatures before V1C6 would be a total waste.
I suggest only maxing creatures related to the creatures SQ and leave the unrelated ones alone. Another tip; get your un-maxed creatures on expedition and stack on those XPs for until you reach V1C6 for a massive level up in one-go. Or if you use GG, leave at least one un-maxed creature (not related to any creature SQ) and tweak the level when you reach V1C6 in the future.
Lastly, Magizoology Rank and Creature Reserve Rank are two different things. Just in case you might've mixed them up.