There are two schools of thought on them that I've seen.
1) Featherfin synodontis cannot be kept with other featherfins, however, it can be kept with other synodontis species.
2) Featherfins should be kept in groups.
Pick your poison.
I keep only one due to tank size and also because I've read in numerous places that rule #1 applies but have rarely seen that rule #2 applies. I also feel that he makes a splendid sole specimen.