There are plenty of cichlids that would work in a 30 gallon, just as there are plenty that won't. I'd avoid Mbuna and any of the larger African rift lake cichlids. Tanganyikan shell dwellers would be fine there, or you could keep some African riverine cichlids like kribs (there are Pelvicachromis occasionally available in the hobby other than pulcher), or buffalo heads (Steatocranus casuarius and tinanti more commonly found than others).
Or, you could go Central American and keep some interesting cichlids. Smaller central Americans like the rainbow cichlid (Herotilapia multispinosa), Cichlasoma dimerus, Cryptoheros cutteri, etc. could be kept in pairs in your tank.
Or, you could go South American and keep Apistogramma (many species are now found in the hobby), Rams (ramirezi or altispinosa are both good choices, altispinosa is much easier to keep), keyholes (Creithracara maronii), angelfish, etc.