Need help on tankmates.

At the lfs they had a brown knife fish and a striped peacock eel. I was wondering if I could add one (or both) to my tank. I already have an African Butterfly fish, and that's about it other that 10 comet goldies left over from cycling the tank. Here's some info on the tank:

Size: 55 gal
Substrate: mixed shells and stones
Temp: 73-74
Hiding places: one behind a rock
any other information needed can be provided

If you remove the Comets, then you can have both. Although it is an interesting mix between the three.