I've always loved South American cichlids. Only problem is in a 40 gal you'll have to be very careful with compatability issues. So much personality variation in each species its impossible to say what combinations will work. In my experience, fish that are similar will fight more than fish that are very different. For example, jack dempseys and jewelfish have never gotten along for me , but dempseys and severums, or jewels and convicts have been fine.
Main thing is the personality of the individual fish. Some SA cichlids are totaly intolerant of any other tankmates while others of the same species will coexist with anything. I have a large dempsey that is best friends with a small female convict. You just can't tell till you observe them in your tank.
Goodluck with your pets.