Are your GTs skittish, nervous, or shy? These are generally reasons for getting "dither fish". If this is the case though with larger cichlids, the dithers need be fairly large. Balas would probably work but would need to be in the 6" range. Tin foil barbs(5-6") would work too, although in your 55g you are pretty much maxed on stocking.
With your breeding Jags, a more proper term may be target fish. If you go with large schools the Jags won't be able to focus on one fish and this will make the survival rate of the "targets" much higher. You may want to try 10-12 tinfoil barbs with your Jags, although eventually your gonna be maxing out that 125 with the pair of jags.
You will just have to experiment, and don't get upset if you lose some fish.