First of all, I would encourage You to get a larger tank. Senegalus can easily grow to 10-12inch, which is quite large, if You consider, that it needs hideouts as well, and that You want other fish together with it.
Depending on the character, they can be very easy, don't touch a neon or they can kill everything, even an ancistrus, and killing a swimming cactus is not that easy. Most bichirs will only kill other large inhabitants when stressed. To decrease stress You should offer them hidingplaces and space.
If You have one hidingplace, one senegalus and one ancistrus for example, they will fight for the one hidingplace, and finally, one day, the ancistrus will loose.
BG
Thomas