Is your use of 'wolf' and insult or an affectionate mode of address? That is the first clue as to whether it fits your characters. If it is meant as an insult, then it would be the allusion to his animalistic nature on the full moon, and if it is affectionate then it would be the same as me calling my kid a poodle, or little bear. Doesn't matter what conventional fanfiction rules are -- you make your own world here!
My own way to write Snape includes having him using insulting terms such as 'beast', 'animal' and 'creature' to put Lupin down (he's a bastard), thawing out with 'silly creature' and 'furry' whatever later on ;-)