Let's see, tiger barbs get very large, up to 3" a piece full grown. You're proposing that it would be A-OK to put a school (let's say 5 of them to be on the small side) that's 15" of aggressive fish in a small 20g tank!!! Yes, having length is important, but a 20g is really just too small for such active and aggressive fish. Some people have better luck than others with these guys. Some have happy schools that get along well together with no issues, others have schools where one kills off the others one by one. Why take the chance? If the kid watches all his fish die off one by one with a bully tiger in the tank, what fun is that? Would he really want to try again after a first fish experience like that?