If you're not inclined to add a third species, then don't. The less species in a tank, the better for aggression. You'll end up with a limit of 2 or 3 males Saulosi in the tank, and you'll be lucky if 2 get their full color.
The number of mbuna also depends on how diligent you are with maintenance. 20 or even 25 mbuna will do well ina 55, but be prepared for a messy tank - it requires a rigid routine and above par filtration. I'd go for the 15 mark if you prefer a more relaxed routine. Neither of these species is overly aggressive, so severe overcrowding in't necessary.
The only thing I'd like to express as a concern is diet. Labs are insectivores, while saulosi are omnivores leaning toward herbivores, and are fairly susceptable to bloat.
As far as bottom feeders, I feel they're a great addition to any tank, but again, don't feel you need to, there's never leftover food in an mbuna setup. Also keep in mind that it's not easy to cator to predatory catfish and herbivorous mbuna in the same tank.
I also recommend avoiding plecos, so that algae can grow on the rocks - algae grazing is one of the natural behaviours of these fish, and keeps them occupied with something other then fighting and breeding.
Good luck.