Dithers are not always for shy fish, they are used as target fish also to re-direct agression from eachother and also to trigger spawning as it sets off 'defence mode'
Also they are good as 'extra fish' because they are to fast for the agressive fish to catch.
A green Terror, jack dempsey or Texas would fit nicely, all of which are on parr agression wise with Cons and Jewels.
People underestimate cons and Jewels because of there size, but they are both probably more agressive than both GT's and JD's...but less so than Texas.
If either the cons or Jewels breed (not with eachother, its not possible) then any tank mates other than Dithers will be fodder...a adult male Green Terror could not take a adult breedign males attacks from either species and pound 4 pound, the two fish you own would make mincemeat of most other fish.
People say 'cons are not that bad' those people have never kept a adult male, thye call them 'mini-midas' for a reason, and i have heard storys of adult males cons taking down adult Jaguars when breeding.
I think you should look into 'Tilapia Butterkoferi' (Hornet Cichlid) they are awesome looking fish and a lot tougher than both jewels and Cons....they get to about 12" of nastiness.