The problem you have is that the male con is established in the tank, gets a reasonable size and can be mighty aggressive even without a breeding partner and all in quite a small tank. You will struggle to add more cichlids I think. If it was a female you would stand a much better chance. I think you could add some tiger barbs and a dwarf plec, but when you add them, take the con out first and place in a bucket. Move all the tank decorations around and add the other fish. A couple of hours later ( to let the other fish settle a bit first), re introduce the con. You should be ok, but no guarantees. Tiger barbs are best in 8+ but you might be best keeping to 6, that's up to you. It sounds like a tallish tank so hopefully the barbs will swim a little higher in the water than the con, and they actually might be good for him to make him a little more confident to swim around. A bristlenose could work as well but get one of well over 3" and try to create a couple of good hiding places in case the con gets a bit too close for comfort initially.
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