I say you test out a few silvers first. I have 3 silvers all around 11 inches in one big tank together, and they get along perfectly aside from a few nudges of aggression every once in a blue moon. But the best reason for starting out with silver arowanas is that they are the cheapest kind available, and they are beautiful as well (especially when they have that shine / pink tint on their fins). Also, they grow the largest (the idea that they will get HUGE makes them more cool to me).
Try those out, you can always find people who would be more than happy to take oversized silver arowanas. Then you can try out different kinds