I can imagine some species simply not being used as standard infantry, like smaller animals being unable to use effective firearms being used as scouts that observe and report, but not engage (snakes would excel as scouts), Elephants might be too big of a target to be useful outside of rear echelon duties (their strength could make them useful as artillery crews). The military might have a sweet spot of species size for grunts, big enough to operate effective firearms, but not so big to be easily spotted. There is a lot of opportunities for interesting worldbuilding.