Depends a whole lot on what you mean about nice, and with what. I have a tank full of ellioti (if anyone wants any.....) that are happily coexisting, until some pair off that is.....
Severums tend to be more placid, but in my experience of them, they're probably more capable of doing damage, but less likely to. So with other big fish, the firemouth every time, for small fish, the severums.