For saltwater folks, it will be cool to throw in a few freshwater fish, as many already do with adding livebearers to control algae. But for freshwater folks to set up a saltwater tank to house cichlids, it's just not practical though it's cool to see others do it.
Although saltwater fish are a lot more colorful and variable than freshwater fish, I heard that they have a tendency to lose color with age in captivity. For freshwater fish, you can see the transformation of colorless juvies into colorful adults. In saltwater, you see the reverse. Also, I have never heard that saltwater fish breed in captivity, so the fun of watching courtship, breeding and fry raising behavior common in freshwater fish is lacking. Some tank buster cichlid are glass banger. Never heard of saltwater fish have a personality.
I have never kept saltwater fish, so I don't know what I heard above are facts or stereotype.