Species has very little importance if your pet is only used as for arms, so long as it has a desirable growth and element. FYI, my FL-JP warrior uses a golden rat as his arms pet, so pick whatever species you want when using nutirents or life core.
As far as what's best, it depends. Sword Sages can get away with using the token fire pets, as their Draco skill neutralizes whatever element is used in arms. For the other warriors, light/dark pets that are merged to grow high STR are generally preferred for the fact that they aren't resisted by anything.
Retired from FL-EN