Gammerus technically arent shrimp, but amphipods, and while technically such organisms would comprise a large part of a substrate feeding cichlid or catfishs diet, they would eat many species with differring gut contents, and as a scavenger each species would have a very varied gut content and mineral balance anyway. Not so the commercially raised ones though, chances are they recieve a meal based food, so I wouldnt make them more than say 5-10% of the diet max.
For your species, doradids, firemouths, (acaras to come) the balance should probably be roughly...
40% good sinking cichlid formula, sometimes split with tetra prima, 5% prawns , 10% chronomid midge larvae,mix and match bloodworm, glassworm, blackworm etc. 10% worms and tubifex etc, 10% fish pieces, 5% shellfish like mussel, 10% veg or an algae wafer substitute or veg based catfish pellets, 10% can be microfoods like daphnia etc, or you could spit that 5% microfoods, and you could make the last 5% the amphipods.
So as a community from the dietary point of view the firemouth /acara/doradid combo will work fine, they all eat similar foods in similar proportions, though obviously they will pick favourites. Probably find the acaras go for the fish and prawns first,then the cichlid pellets, the firemouths will go for worms and sift small foods, and probably hit the prima, the doradids wont be fussy, but will probably take most of the wafers pellets, and go for worms too. In that kind of company all three species should self regulate diet and take what they need , and stay in good condition.
The convicts would take the same except you nudge down the other proportions and make at least 20% of the diet blanched veg or veg formulas. Obviously they wont be in that community for sheer violence reasons so they would be fed seperately anyway.
So basically treat gammarus as if they were another meaty treat item, and dont let it dominate the diet. All useful dietary additions though, variety is the spice of life and key to health. On that kind of diet and with decent filtration and tankspace there wont be a dwarfed fish in the house, they will probably all breed and live a long time as long as not overfed, and heal from scrapes and injuries at lightening speed.
If you wanted additional vitamin c in the diet its probably better done with fish vitamins and sprays on the food than with actual fruit with this gang of protien loving hooligans, most of em wont look at fruit twice apart from maybe the convicts.
If you end up with a GT in the future they basically eat the same as a blue acara but the size difference will mean they go for a slightly higher percentage of shrimp and fish, being more capable hunters.