I’ve had a look around online, as this is not a question I can reliably answer, but I have discovered on a specialist goldfish site that there are issues involving the size of tank you want to put them in – apparently they are quite dirty among the fish world. Also, the best comment seemed to be that if one fish is small enough to fit into another’s mouth it will eventually end up as food, eek!
Welcome to Fluther @fish_horses – don’t mind our humour on here, we’re a sweet bunch really! (Questions in Social often end up with little puns in – if you’re really after very specific answers you could try asking in General?)