Hmm.. your tank is a bunch of pretty melow guys with the front... You are probably safest staying with another peacock. Many haps are also melow, but most get large for that tank, especially with what you already have.
How about a couple of catfish. I have Synodontis Lucippinis. They are really cool fish and get to be about 3 1/2 inches only. They are from Lake Tanganika. And, they are pretty rare!
Otherwise all the fish I would consider adding are very pretty, but not all that rare. Yellow labs may work, although their temperment can vary (the one I have now is rather agressive, but I have had some very sweet ones in the past.) But with what you have int hat tank, Catfish might be a safer choice.
On a different note, that is a rather small tank for a front.