Many kind of vampire tetras seem to be claustrophobic and cannot be in smaller tanks like cichlids.
I say in general you want to have twice the vampire tetra's length just for turning space. Sometimes though, a 12" payara can work out in a 18" wide tank.
A scomb could stay in there about a year before it freaks out and breaks its nose, A Tat or "Red Tail" would work for about 6-7 months depending on how much you feed. And Armatus are out of the question.