In my opinion, it really depends on the species. Some snakes, such as ball pythons, do not do well when housed together, while others, such as green tree pythons, do. Some snakes when housed together will try do dominate the others, or fight over the basking area, while others do not. Snakes housed together may also pass illness, disease, and / or parasites to their cage-mates.

As for Rainbow Boas, I have never kept them, so I can’t answer your question one way or the other.