I think that everyone so far is in agreement that it is a poor mix, the problems you will be facing is not only aggression, which could partly be handled by just getting a tank big enough.
In my opinion the main problem with any sort of mix is needs of the fish that you mix. Not knowing what sort of african cichlids that you have, it is hard to tell if you can achieve a balance with the other species, you should always start out by considering the following:
1. Water quality, does the mixed species prefer the same kind of water quality. In you case the Jacks are likely to prefer a much lower PH than your africans if they are from the rift vally. You can balance it somewhere in the middle and provide a water quality which none of your fish prefers but which they can all survive in.
2. Environment, does the mixed species prefer the same levels of circulation, the same substrate and do they have the same needs for tank design. Again in your case the "americans" will prefer slow to medium circulation and some decor that allows them to "stand" and hide or dodge quickly thru some branches. The africans are likely to prefer high volume circulation and would want some caves and rockformations where they can hide and spawn.
3. Food, you need to ensure that the mixed species can survive (at the very least) on the same diet. So if the africans that you have bought are herbivores, how are you going to make sure that they do not try to eat the beef heart or blood worms that you will feed the americans. The herbivore digestion system are not geared towards the high proteion meat diet of the american cichlids, and they can develop serious problems such as bloat if not fed a "green" diet.
4. Behavior, the mixed species should have similar behavior patterns to avoid stressing the tank mates. Stressed fish are much more prone to parasitic illness and are less likely to express "natural" behavior. Again in your case not knowing which africans you have it is hard to tell. But it is likely that the behavioral pattern is very different from the americans. Fish who in the natural environment is "standing" and moving slowly around in search of prey will be stressed by a bunch of rock beating africans constantly darting all over the place.
I am not against mixed tanks, but you have to consider the 4 basic things if you want to care for the fish and try to fullfill some of the basic needs.
Would you put a cow and a tiger in the same small pen ? Even if you could avoid one killing the other it is very hard to imagine that they would thrive. Which is why agression is not the only factor that you should consider.
Find some fish that have the same basic requirements and you will be much better of. You can check the web or this forum to read the basic requirements for the species you want to mix and then go from there.