You can buy GH & KH test kits and test the water at home.
What are the tank dimensions (length x width x height)?
Mbuna are pretty territorial and should either be kept in breeding pairs or colonies consisting of 1 male and a group of females, or in a single sex tank of either males or females. If you have males and females the males will fight more to get the females. If there are only males or only females, they will be less inclined to fight over partners.
If you buy a group of young fish and grow them up together, they will settle into the tank and develop their own pecking order, and they should be fine.
If you have a couple of fish in the tank and add new fish, the new fish will probably be considered intruders and get attacked. If you do want to add new fish to a tank that already has Mbuna in, you should rearrange the ornaments to break up the territories and then add the new fish straight after you moved the decorations around.