Festivums should be fine with rainbows and dwarf cichlids. They're very peaceful, and while they are about as predatory as angels, in the sense of happily eating neon-sized fish given the chance, they will be fine with rainbowfish. Festivums are lovely cichlids, and great characters.
Severums are basically similar, but breeding pairs at least can be quite aggressive.
Both species are herbivores in the wild, eating a lot of plant material alongside the usual insect larvae. So you'll need to offer a variety of foods to keep them both healthy. Lots of people neglect the plant aspect of their diet, and then wonder why their fish lose colour. Duckweed is a favourite, but Sushi Nori, vegetarian flake food, thinly sliced courgette, spinach, etc all work well. Don't give either species high-protein foods like beef heart. There's reasonably good anecdotal evidence that giving herbivorous cichlids meaty foods causes serious health problems. While these are unlike to be as bad as, say, bloating in Tropheus, it's not worth the risk. Obviously, they will view a planted aquarium as a buffet, so choose any aquatic plants with care.
Cheers, Neale