buffaloheads are too aggressive to be with rams, although you could do buffaloheads and barbs.
For mixing the continents, you can sometimes mix non-rift lake africans like jewels, tilapia, etc in new world tanks. I have a Paratilapia with my new world cichlids. you can mix kribs with smaller acaras for example. Buffaloheads do fine with smaller CA's. I wouldn't recommend it, but I've seen malawi haps mixed in with CA. I imagine if the water suits the malawi's you could theoretically have CA that are acclimated to it as well.