Red bellies aren't really aggressive. They're just giant tetras and, like many of their smaller cousins, are predatory. Aggressive, territorial fish will probably bully a red belly and kill it. The RBP might also take an occasional chunk out of another fish because it'll be out-competed for food and get desperate. I've kept RBPs in schools with a variety of large, armored catfish before, plus plenty of smaller dithers, shrimp, etc. to keep them occupied. Even then, I did end up with one RBP and one catfish (over many years) with chunks taken out of them. Both were QT'd and healed successfully, but it's always a risk.