Not sure if we get any others, but a leopard ctenopoma would be an ideal addition to an african riverine tank, as would a school of congo tetras. Buffalo heads are another possibility, but I'm not sure how fast they grow, so you may want to get them at a large enough size so they don't get eaten as the senegal grows. I'd recommend getting a few senegals for a tank that size, 3+ would be good.
And if you haven't already seen it, Annes Polypterus Primer should answer almost every question you have: http://www.monsterfishkeepers.com/forum ... hp?t=65770