Keyhole cichlids might work, I hear they are pretty peaceful... I think the majority of cichlids though get fairly large and somewhat aggressive, your smaller fish might be in trouble. I would definately hold off on adding anymore fish for at least a couple of weeks after you have stable test results. I still am a bit baffled that a weekend of overfeeding caused such extreme problems....
You've got lots of room in your tank, I would round out what you have before adding any different kinds of fish. For instance, I would go with 4-6 more rasboras, 4-6 more zebras, and at least 2-4 more corys (of the same kind) if that is indeed what you have. Since those are all schooling fish, the bigger the groups, the more likely they are to hang out together, which looks pretty cool. Maybe a couple more platys? I wish I had less species of fish, and just more of each, now that I am pretty much done stocking.
You had some pretty rough test results for a while there, it might have just been too much for the fish, which could be why another one died. It sucks, but I wouldn't worry about it too much...
