They are less aggressive, and usually a tad smaller, also they are weaker fish from the inbreeding... So they are harder to keep and the smaller one's can randomly die from unknown complications.
Yeah they are rumored to be weaker, I don't know for sure. I've got 7 of them growing out now in my 55gallon, beautiful fish.
I wouldn't suggest them unless you are going to be very strict about water changes and all that good stuff. I do about 25% - 35% change every 3-5 days. Also, know where you get it from, there are a few online sources that are VERY reputable for thier stock quality.
That tank is stocked. IMO you shouldn't add any more cichlids. Maybe some dithers or something if you feel you absolutely must add something new...my 2.