Severum do well with bichirs I have an orange one. I like ctenopoma I have one in with my bichirs. Be careful with angels because I had an ornate that was a fin nipper. Also had large electric blues with my bichirs for a while. Dats do well to, be careful with the catfish idea, I found it is better to let bichirs be the only bottom dweller because I had a synodotis that was pretty territorial. Plecos are a no go.. mine always ended up sooner or later eating the slime coat and I've tried several species..
Good luck man, bichirs are fun to keep.
How many you can keep in a 120 is up to debate, water changes are real important, but they do like to pile up when not active in my tanks no matter what the cruising space or how many I have. lol Mine do hang out by species for the most part.. delhezi pile.. palmas complex and moke pile.. teugulsi has his spot. Retros have theirs.. It is pretty funny. I moved out my syno cat because he would just switch his favorite area and my fish would be so stressed so it was for the best to leave him out.