I agree, for the most part that is true. There are always cases of people mixing and it working out. If your gonna try it, I'd look for a more laid back mbuna IMO Acei are usually more laid back than let's say afras. As far as peacocks from what I've read, and some one may know better than me on this, but I think the jacobfreibergi are a little more robust as opposed to some other peacocks. You can probably make it work if you want. Just read up on what your interested in and if you have any questions on specific species mixing there is a huge wealth of knowledge on the forum. One of the biggest things to help aggression is rock work and caves.