What should I do with my cichlid?

So a few months ago I got a wild hair and ditched my old 10 gallon for a shiny new 20 gallon. Along with that i figured why not go ahead and get some African Cichlids(a had them before but was never really successful) I skipped on over to my favorite pet store and picked up:

1 Red Zebra Cichlid

1 Yellow Lab Cichlid

1 Dragons-blood Peacock Cichlid

2 Silver-moon gouramis

1 corey cat fish

I know this is very over stocked, but I read that was what you were meant to do with mbunas. Everything was going great, and all of the fish were very peaceful with each other. I didn't have a single fatality for a week and a half. Then I got some Christmas tree moss off of ebay and it through the chemicals in my tank wayyyyyyyy off. The nitrates and ammonia were off the charts! I immediately got anti-ammonia tablets from my pet store and plopped em in. I also vacuumed the gravel and got it mostly under control. Then it shot right back up and killed all of the fish but the Red Zebra and the Corey cat!

I want back to the store for more chemicals and Wick, the store owner, told me to try some dead rock. That stuff worked wonders and I soon got two more peacock cichlids (thats all the store has in stock). one died of shock and the other was very healthy. Up to this poin I had thought that the zebra was a male because it is very colorful, has several yellow spots on its back fin, and was the dominate in the tank before. I had also seen it try mouth fight with its reflection. But to my shock the zebra came up holding one night. I sprang into action and took the male out of the tank and put him into a 5 gallon i have, which killed him from the shock(I have stacked stone in my tank so it was like WWIII trying to get him in the net). The zebra also swallowed her eggs(you can only imagin the amount of curse words left my mouth), so i got her good and healthy again and went back(yet again) to the pet store and got two more peacocks and a common pleco, who is odly enough now my favorite fish in the tank. (there was a substantial amount of algae on the stacked stone at this point.)

I put the three new guys in, and everything was ok for about an hour, then my red zebra got blood lust. She killed the larger peacock(I ided hi as a male) and now has the smaller peacock(Ided as a female) trapped under a rock where the zebra cant get to her.

These fish are getting expensive at 10 bucks a pop, and im trying to get a job at the pet store so it looks bad if I keep buying fish.

I need help on what I should do I want at least 3 fish in my tank or it will be boring as hell.