Congo Tetras (Phenacogrammus interruptus) will be fine with African Butterfly Fish but, as I'm sure you're aware, both types of fish will outgrow your 10 gallon tank. Perhaps you could get a 20 gallon long tank...which would supply extra swimming room for the Congo Tetras and provide plenty of surface area for the African Butterfly Fish. Toss a small Synodontis species of catfish in there and you'd have a nice little African tank with activity at all levels.
Personally I'd avoid any kind of crab/lobster/crayfish/prawn. If you really want an invertebrate then the Wood Shrimp (Atyopsis species) would be your best bet. They're filter feeders so they're no danger to your fish and they get large enough that fish like Congo Tetras and Butterfly Fish will leave them alone.
-Joe