No matter what the species or the source purchased from you need to quarantine any incoming snake from your established and healthy snake. Having a mixed collection only makes that more important.

As far as housing you certainly don't need a massive tank or tub for a baby boa. A nice smaller tank or tub will do quite fine for the baby, let it move about and yet still feel safe and secure. RTB's are wonderful snakes but do grow fairly large so if you aren't wanting a quite big one stick with a male (make sure that's confirmed through proper popping or probing).