Corn Snake Vs. Fuzzy?

ok i have a around 9 month old corn snake.. and i was feeding her 3 pinkies a week.. then it was 4 ever week and a hafe.. then i feed her a fuzzy.. i was in the petstore and we were having a big argument how my snake shouldnt be eating fuzzy's yet.. if it was to big for the snake wouldnt it just refuse to eat it because most snakes size there meals.. anyways it ate the fuzzy really slow manly because my corn snakes head has not grown but his body has.. he was fine with it.. i did make a mistake on feeding him to soon afterwards cos i fergot when i fed him.. i should have waiting way longer and the second mouse he threw up ( couple of ays later) but it was almost gone.. anyways the pet store said i can kill my snake.. and they got me all worried.. if he can eat it it shouldnt be a big deal right? maybe he is just bigger then most cornsnakes at that age?