28 fish at that size is not overstocked. When they grow up, you will have to move some fish out. The amount of fish needing to be rehomed depends on the species you keep, the size of your tank (common 6' tanks could be 125, 150 or 180) and its filtration. What is your setup?
Most haps get to be 8"-10" or longer. Even the adult peacocks in my 180 are all around 6" long. For older adult fish, about 15-18 is pushing the envelope on a 180, even with excellent filtration (wet/dry or FX5/AC110 combo). You will also need to do weekly 50%-75% water changes or else bad things start to pop up very quickly.
Rocks are good. For a large Malawi fish, THR is a waste imo as it takes up considerable tank volume and they are too big to fit in the holes. It is great for smaller fish and fry to hide it, but you are doing all males. Go with large pieces of slate stacked to create a few large caves. Don't go nuts as the open space will benefit them more.
Given good water quality and feeding, your fish are going to grow up fast.
Check out the movie of my 180 Malawi setup in my signature. This is from a couple of months back and I've since moved a few fish out. They see me at the front of the glass and are looking for food, so it appears more stocked than it is... but this is maximum density I'd feel all right with. Note that many of these fish are not full grown (the Fossochromis and the Stigmatochromis are in there to grow out) and will have to be moved over to the 150, a breeder tank or sold off.
Eventually, I will go down to about 15 or so fish in the setup.