My red minor serpae tetras are quite nippy. They harassed a couple of mollies relentlessly. Had to move the mollies to another tank but they never quite recovered. My pristilla xray tetras are much calmer and very pretty...school tightly and the silver shimmers in low light. I don't know how they'd be with what you already have but they've been super peaceful in my community tank. The serpaes nip each other too. I'd definitely avoid them with a betta but maybe someone else knows if the xrays would work?