Not trying to be harsh but if i were you, I would get rid of The balla sharks and danios and catfish. As your cichlids get larger they will get more aggresive. Now with that said, I would look at (Mbuna's all from lake Malawi) Like (jeaninel) has stated. Your cichlids will get up to 5to 7 inch.
Your pleco depending on what kind it is, will get way to big for a 55 gallon. If its a common pleco.
Your bala sharks will get to big for your tank size.
Your catfish will get beat up by the kenyi's and red zebra.
There are plenty of cichlids you can put in your tank.
Your water is going to differ for the differnt speics you have in your tank now. Your cichlids will need a ph of 7.8 to up to 9.0 so i would start with that. I keep my ph at 8.2.
Here is a web site to look at what kinds they have for sale. Livefishdirect.com They all so will sex the 2 to 3 inch ones for you to the best of there ability. I hope this helps. If you have any concerns just ask we will gladly help you.
Jeaninel has allready stated on the diet for you. You can also give them bloodworms once a week or every other week.