Definitely agree that shelldwellers, especially multies are worth a shot, they are extremely interesting in terms of behavior and they are also easy to breed.
if the fish in question are mbuna you definitely shouldn't feed them fish. Honestly, I don't feel comfortable feeding that to any of my fish, and it also tends to be messy.
The big stores usually have little ID labels on each tank with a picture of the fish (at least Petco does), so either the customer or the sales associate should've had the wisdom to just take a quick glance at it.
rosy reds shouldn't make up for any significant portion of their diet though. I would use a quality pellet food and a variety of frozen (or live) foods.
depending on the size of the tank, there are some you can mix. I'm sure you can have a pair of rams and a pair of L. curviceps (I'm not sure if you mean festivum or curviceps b/c I've seen both labeled as such) in a 55 gallon.