Could i keep 3Melanochromis auratus and 3 Cynotilapia afra in a 30gallon.. I have a bigger tank for them when they get bigger...
You're going to want to start with bigger groups of each to diffuse aggression. Afra are lower on the aggression scale and even they won't work with only 3 fish. Auratus are a whole 'nother can of worms. I'd recommend starting with at least 8-10 to get a good final ratio (maybe 1M/6+F as adults).
That being the case, you could keep them in a 30 for a little while (very little while), but they will need more room by the time they hit about 2" and aggression kicks in. The added bioload of this many fish in a 30 will also be a concern.
If you're looking for something that fits nicely in a 30, try a large group of P. salousi.
Good luck.
In a 30gallon I would probably stick with one species. If you are like me you probably want a variety of color, which makes the P. saulosi a prime candidate. They are dimorphic, with males being blue with black bars while females are a nice yellow. They are also a smaller species, maxing at about 4". The only thing is that they can be aggressive so you will want a good ratio of males to females. I would suggest going for 2 males and 5 or 6 females. You should always try for a 1M:3F ratio with any mbuna species. Also make sure to have lots of rock work and hiding places for the non-dominant to hide.
One other thing, when P. saulosi are young they are all yellow. As the males mature the turn blue, but holding females can sometimes display male colors as well. Here is a website that can give some information on this species, as well as others.
For the P. saulosi:
http://cichlid-forum.com/profiles/species.php?id=1
For other species:
http://cichlid-forum.com/profiles/