If you're sticking with peacocks and small to medium haps (non-borleyi Copadichromis, Otopharynx, Lethrinops, blue Placidochrmis, Scianochromis fryeri, 6" max stuff), you could probably be fine with 15-20 or so adult fish in the end. If you are doing bigger boys (Protomelas, Nimbochromis, Fossochromis, Champso, Tyrano, etc), it will be less. Note that this will take a long time to get them to be adults (a coupe of years from 2"). In the mean time, you could pack out the tank... just make sure you have excellent filtration and weekly water changes.
Right now, I have about 30 or so growout peacocks, mbuna and haps in my 150 (a tall 125). They are all about 3"-4". Eventually, I will have to reduce the number of fish as they get big, but this will be a while.
In the meantime, have fun. Just stay away from more large haps (beyond the Red Empress and the Taiwan Reef you already have... they max out around 8") and be prepared to get rid of some fish down the road (or setup another 6 footer!).