Some fish are aggressive, always spoiling for a fight, can't get along with others.
Others are merely predatory; anything small enough to fit into their mouths...will wind up there.
Salvinis are both! And although they don't grow huge, they can still hit 8 or more inches; in a 75-gallon tank you may have issues with just keeping the two of them together when they mature, unless they happen to pair up. I kept them only once, and they were impossibly aggressive, in a much larger tank. I doubt that you will be able to keep the bichirs with them long-term; adding anything else is, IMHO, a pipe dream.