Greens, Golds and Reds are the biggest as they are all the same species Heros Efasticous - where as Heros Severus (the mouthbrooder) and Heros sp.Rotkiel stay smaller and would be pretty ideal for this size tank
I have a female Heros Severus, shes called Lilly this is an old picture of her now, she is about 10 years old and 8 inches. They have a great red colouration and sometimes she gets a blue sheen to her.
Severums are one of my favorite fish
I like the idea of mixing them with the Bolivian Rams, I have Cupid Cichlids with mine and that works well and I imagine its a similar relationship in the tank. I would think about the Rosy Barbs only because I think that the tank would better suit some kind of Tetra - Lemon Tetras would work really nice as would Flame Tetras their behavior is much more natural for the cichlids and their calm nature will bring out the confidence in the Bolivian Rams. You could also look at some larger Characins like Spotted Headstanders which would work nicely
One thing I would say as well is about getting some bottom dwellers in there - Whiptail Catfish are really nice and you can get some great species that are really graceful, I like them because they are more active than normal plecs and are usually out all day. Some kind of bottom dwelling catfish like Cories and Hoplos, Dwarf Hoplos are nice but I also like tanks where they have a school of cories and a trio of normal Hoplos I just love the little and large look
Hope thats helped and given some ideas
Wills