As dithers I have used:
-
Epiplatys dageti with my
Pelvicachromis pulcher "Red form" which are "kribs" in a 29g:

the killies have spawned as well as the cichlids
-Montezuma swordtails with
Pelvicachromis taeniatus "Lokundje" in a 20L
-Endler's Livebearers with
P. taeniatus "Keinke" in a 20L.
A dither fish needs to be a disimilar fish (ie; not another cichlid or benthic dwelling fish) and a fish that can take a little "run around". Apistos qualify as neither and honeslty, I would take them out ASAP to keep each species from getting stressed (ie; the apistos as they will lose any battle to a pelvic, expecially a pair) and possibly sick.
Also, I would suggest keeping your Pelvics in no smaller than a 20L. Sure, while they are acclimating, they'll be fine in anything (even a bucket), but you WILL come across aggression issues eventually. Even my pair bonded fish will harass eachother if there is not enough "personal space". I keep plants in my Pelvic breeding tanks as well as caves for just this reason.
If you just bought "
Pelvicachromis taeniatus", you overpaid. $40 a pair should come with a location varient name. Resell value on a species like this without a location name (ie; "Lokundje", "Keinke") won't be much; I'd say as much as a common kribensis.
As for food, like was said, they'll eat through detritus, but any frozen food given in variety should suffice.
GL. HTH.