Unless your BP can actually close it's mouth (most are so deformed they cannot close their mouths) there is little damage a larger BP can do to an angel (it could try to ram it but unless you get a stupid angel who sits there and lets it happen it likely won't harm the angel). If you get an angel the size of a 50 cent piece, after qt I'd be putting it in the 55g. It's not like you're getting a dime sized one, or even nickel sized.