I've kept my Sen with a ton of different tank mates over the decade I've kept him: false gars (needle nose, ect), tinfoil barbs, sleeper goby, oscar, loaches, and currently knife fish (brown and golden), and a scomb.
The only things that HAVEN'T worked with my bichir were:
1. fish small enough to fit in his mouth..... I've lost many small to medium sized tetras to this fish
2. Plecos - I only kept him with one, and it started sucking off his slime coat