You would be surprized, its funny how you can get fish to be friendly with each other.. I would know, i have gold fish in one of my tanks, and my green terror, oscar, zebra, or sunrise peacock fight with any, its really surprizing
In my opther tank a 30 gallon, i have a small frazona, to eletric yellow and a red tail they all get along fine. Ive always added new fish in the dark @ night if that makes a diffeence