Don’t introduce predators to prey. Make sure no fish is small enough to fit into the mouth of the largest fish. Not that many fish commonly kept will kill other fish and eat them unless they can easily swallow them, even most of your typical cichlids. The main concern is whether a given fish can fit into the mouth of the biggest fish. If a given fish can swim along and easily just eat the other fish, then mixing those fish isn`t a good idea; if he can’t easily kill the fish, you`re normally OK. So you just have to make sure your biggest fish can’t eat the smallest fish in the tank. Some fish are very aggressive and will kill other fish for food or territoriality but this is not common. Be aware of fish like Red Devils.

As per.com Paul Talbot
Taken from www.cichlidhub.com