It does look like a synodontis decorus. I have one and they are quite shy.Firstly you have to take onto account that they're nocturnal. Provide plenty of hiding places ,rocks tubes wood etc and once they become more confident they will come out to feed. They may be shy because of other tank members, so what are you keeping them with. Syno decorus also get pretty big upto 9-10 inches which doesn't look much on a ruler but when you turn it into fish is pretty big HTH
You've got a bit of a mix going on there, None of the fish you've got should really bother the synos though could I suggest you get the comet moved as it shouldn't really be kept in with tropical fish and will also grow quite large. The other fish will also be at risk as the decorus grow, as the will eat smaller fish. What size tank are you keeping them in and with what filtration? Could I suggest you go back to the shop you bought them from to confirm what they are.
I think you need to go and confirm what this fish is as a 22" tank is not big enough as it grows, unfortunately height is not too important. If it is a decorus I would recommend that you either remove it soon or best idea, get a bigger tank