I currently have a senegal and a delhezi and plants with a Kribensis, which can be a very colorful fish. They have not had any fights but it may be a problem once the bichirs are fully grown, but I hoped to minimize that possibility by introducing them all together when they were very young.
I also have an African Butterflyfish that is very interesting to look at, but not very active in general since it stays completely within the top 2 inches of water. At feeding time it is extremely active though.
Even though I think most people around here will tell you to get more bichirs (which, don't get me wrong, is a great suggestion for your sized tank) there are plenty of other possibilities if you want a more diverse tank, and I'm sure someone else can chime in with their more thorough experience.
On a side not, DO NOT EVER get a Longfin Rosy Barb. I have one still alive amazingly for 5 months now that I introduced when the bichirs and cichlids were young, but it constantly has to regrow its fins due to nipping. It seems like every time the bichirs take a bite, as soon as the fins are gone, they couldn't care less. When they grow back, it's considered food again. I don't understand it...