Tank will be full with adults of what you have. If there are m/f pairs, then territorial instincts will kick in and only one pair will remain. No to keeping African Mbuna with CA cichlids. No to a pleco. A small species may be picked on, a bigger one is too much bioload for 75g holding 4 cichlids.
the last time i had a pair of jacks in a 75 gallon i couldnt put anything else in there with them.and the male eventually killed the female after spawning several times.right now i have a 10 inch red devil in a 75 with several small dithers which he seems to ignore.all attempts to put him in a larger tank with other cichlids has failed.
I think you could get away with one of the smaller pleco species, like a bristlenose or rubber lip.
You could also try adding some live bearers to occupy the mid & top portions of the aquarium.
But honestly, as far as cichlids go, I think your 75g is stocked about right. In fact, as it stands you may have one Jack Dempsey too many, or 2 convicts too many...
I used to have a 90g tank (same footprint) that started with a pair of convicts and a pair of JD's. I ended up with just a pair of JD's, until the male killed the female.
If I was to attempt that tank again I would probably get a breeding pair of convicts and 1 jack dempsey. I believe JD's are piscovores, and the convict pair could supply it with an endless supply of live food.