The red hump Geophagus group (steindachneri, crassilabrus, pellegrini) are not quite as social as some of the other Geophagines, so putting one of those in with your other cichlids might work. The red humps are very aggressive with each other, I had males that would assasenate any none compliant females in 6 ft 100plus gallon tanks, even when small.
I believe as Gourami swami said, you are over estimating the size of your tank, and how many territorial cichlids it is capable of housing safely.
The Acarichthys (thread fins) also get quite large, and at matururity will assassinate each other in 100 plus gallon tanks. I was warned I needed more than 200 gallon tanks with plenty of floor space, to keep a copasetic group, but thought (erroneously) I could handle them. After three years of compatibility, they killed each other off in my 150 gal almost overnight.
The Acarichthys above were about 10" L when the photo was taken.
All cichlids are complex animals, and most new world cichlids do not react well to crowding because of their territorial nature.
Also realize as your cichlids grow, you will need to up your water changes and other maintenance practices to keep water quality good enough for proper health.
Large mature cichlids in an undersize tank, often mean every other day, or even daily large water changes.
