Fish that are similar see each other as competitor. Fish that are dissimilar tend to ignore each another. This is why mixing Africans and CA can achieve greater harmony than all Africans or all CA tank. We all use dither fish to induce calming effect or diffuse aggression, so why not mix in dissimilar CA and African to achieve similar effects.
I agree with much of your post and think it's possible to mix fish from continents if you want to, but don't fully agree with this logic. I think the CA cichlids will still see the Africans as cichlids, and thus competitors, much more than a dither fish. What you're describing are target fish, dither fish would be fish which are
not seen as competitors, and don't diffuse aggression, but make fish feel comfortable, since in the wild (ca cichlids at least) use them to gauge threat from predators above.
For example, a Salvini cichlid looks about as similar to, say, a Red Devil as an African cichlid would- it would be smaller, different body shape, different colors. But, the red devil will know that it's a cichlid and compete with it for dominance. If there were schooling fish like silver dollars in the tank, it would have a much better chance of ignoring them, since it wouldn't see them as competition. I've worked in LFS's for many years now and have had to balance temporary stocking of our cichlid tanks- I've mixed africans and centrals plenty of times in this context, and then done cichlids with non-cichlid dithers. The centrals IME know that the africans are cichlids regardless of color and body shape.
I remember you posting your tank- I'm not meaning to bash you at all, I very much liked your pair of panamensis, since I keep them myself. But if I remember, the tank was a 55 gallon which had lots and lots of cichlids in it, including the large viejas, panamensis, and africans, etc. I think that your success aggression-wise is largely due to how many cichlids were all in such a small tank, so many target fish- and not primarily due to the dissimilarity of the cichlids. Long post, but my point is, I think centrals would absolutely see africans as opponents, and not dither fish.