Utter rubbish! Fish will either grow to their proper size (if they have enough room, food and water changes) or will become stunted and die young (if kept in tanks that are too small, not fed correctly or have insufficient filtration or water changes)I've been told that the catfish type fish only grow depending on how much oxygen is available in the aquarium. Not sure how truthful that is.
I'm sorry; I wish I could be more positive, but I'm 99% sure that is an oto, and they are a very delicate fish that needs specialist care. Not one for inexperienced aquarists, I'm afraid.Without wishing to sound harsh, on it's own, in a new set-up, it's unlikely to last long.
i hope not =[
Most LFS are just after the sale; you can't rely on their advice; very often they don't know and will just make something up

/www.ebay.co.uk/itm/juwel-rio-180-/320741076090?pt=UK_Pet_Supplies_Fish&hash=item4aada86c7a