Armored bichir a.k.a. Delhezi grows up to 14" so you can put several species... To make sure they wont eat each other, just be sure that they are in the same size...
You can feed them silversides or feeder fish BUT I wouldn't recomend it... Coz most feeders carries desiesesfungus... if you still want to, just quarantine your feeder for 3-4weeks before introducing them to your tank...
IMO... Polys will do better and will look good if you keep many of them in one tank...