Yeah they usually wouldn't tell you that because eventually the fish would die from being stunted and then most would just buy a new fish, if I didn't know better that's what I'd do at least. We used to do that when I was a kid too since we didn't know much about them either. But that's okay, you know now ^_^
Yeah, you can try it and well if it doesn't work out....it doesn't work out I guess. But it's better than keeping him crammed in a 10 gallon I guess then right?
Just make sure to acclimate him really well to the water since the chemistry is bound to be different! For the first half of his life he'll eat the algae and stuff that's in there so that's good but as he grows he will eventually start to eat meaty things, goldfish beware when he get's too big! o.o I don't think he'll make it to the full 27cm though, that's pretty darn big!