I probably wouldn't do the salvini as first choice, but if I had to house one in a 37g; I'd have no qualms to be honest.
Fish cruelty is a very ambiguous subject, given that fish cant actually say "hey, you're being cruel to me".
It's all about reading the body language and behaviour of the fish, which is how you'd decide if the tank is too small for the fish. Fish hiding excessively or being skittish, is a key sign to a tank being too small, as well as slow development.
This comes with experience.