Pet guy wrong about fish?

So I went to this pet store(can't remember the name it's in Tuscaloosa) but I went in and there were some plecos in a fish tank(idk what kind) but they were $2.99 and so I said why not go ahead and get one I picked the smallest one from the litter and I have a 10gal tank at home and I asked the guy working "will he get to big for my tank?" and he says that the pleco will grow to fit the tank. Is he right about that or is it going to get oversized and I have to get rid of him somehow?