I'm no expert but from what i know i think they both require different water parameters, also african cichlids are alot more aggressive than the american cichlids, hence the need to "overstock" them to spread aggression.
definiely no. African cichlids require high PH and are more agressive as it was said here.American cichlids live in a lower PH water some species like the discus even require even lower and are very susceptible to any small change in the water.
When it comes to shops always have one thing in your mind they want to make money.If you need info in this day in age don't ask the shop reasearch yourself.