Just to chime in here, adding a demasoni to start the cycle is very very bad, two reasons, fishless cycling with pure ammonia as described here
, will create a far bigger bed of good bacteria than using a fish, if done correctly it allows one to fully stock the tank once the cycle is complete. Second reason is a demansoni is an extremely aggressive fish and if left in the tank alone while cycling will take the whole thing over. Once you add other fish the demansoni will see them as a threat and either mostly kill them or at least make them severely stressed.
I would also consider adding another filter, african cichlids are extremely messy and produce a large amount of waste, you need to look at between 10-15 times the tank volume for filtration at MINIMUM. I cannot stress that enough, the water parameters will decrease very rapidly if not enough filtration is present.
In terms of the other questions, rocks, almost anything, larger rocks though I would add a piece of light diffuser panel(eggcrate) to the bottom of the tank under the substrate sand to act as a cushion should the rocks collapse, cichlids are very good diggers and can easily topple rocks potentially cracking the tank bottom or side. Add some vinegar to any rocks and if it fizzes do not use it.
In keeping the tank over populated yes it will reduce aggression but you also need to house species that can get along in a small tank like that. I would highly recommend you stay away from Demansoni, Auratus, Pseudotropheus Crabo, Salousi and some of the Tropheus variants in a tank that small, the aggression level of these fish is such that in the blink of an eye the entire rest of the tank could be dead with only the dominant male left. They are all extremely aggressive and not suited to a beginner in cichlids.
NEVER EVER, go on the pretense of "I will get a bigger tank", chances are if you cannot afford one now, you wont be getting one anytime soon, sorry but its a fact, we can all say that we want a bigger tank and then something comes along preventing it from happening. Mbuna which is what these fish are, grow quick meaning some can be adult fish in less than a year, above 1.5-2" these fish will start showing their adult traits which can in the space of housing aggressive fish turn into WW3 and dead or injured fish in a matter of hours.
I have kept a tank of aggressive fish and I know what these fish are capable of doing.
I would seriously stick to more sedate species, such as yellow labs but all things aside, you are pushing the limits of minimum tank size to successfully house any Africans in a 29g tank, the shellies would be the best option. It depends on what your water are out the tap which will ultimately determine what you can keep. Buffering is a long drawn out process which can be tiresome and stressful at times not only to you but your fish more importantly, it would be best to stock what is suited to your "out the tap" water conditions.