The sergeant major should be ok with the Cameroon shrimp. They're called armoured shrimp for a reason - it would be very difficult for any loach species to do them much damage. Also you should add some caves/half buried flowerpots so the shrimp can shelter after moulting its shell. Despite their potential size, the shrimp can move extremely quickly if it needs to. I have kept them with Bolivian rams, a red-tail shark and a marbled synodontis and they have never been hassled. In my experience they have only squbbled with ADFs, which is funny but of little threat to either.