I've kept red empress in hap/peacock tanks many times and for me the red empress were always one of the more mellow inhabitants. This also fits any profile I've ever read on them. Like any cichlid I expect there can be exceptions. Also, what you could see is if another fish picks a fight the red empress might finish it.
Like most Africans having just two red empress males, especially if there are females, can be trouble (as opposed to a larger group of one species). Sometimes two male haps of different species or a male hap and male peacock decide they don't like each other-- typically instigated by the peacock ime. So there could be scenarios in which a red empress seems aggressive, but overall not really ime with hap/peacock community tanks.
A single frontosa is not normally an issue at all in a hap/peacock tank. Did it several times before I got more into keeping Cyphotilapia in their own tank. If anything, more likely than the frontosa being a problem, some fronts will be stressed/unhappy in a tank with Malawi scale aggression and energy. Not always, some fronts do just fine in a Malawi tank. I did have one exception with one that did not tolerate any other fronts in his tank, nor any colored up Malawi males, he'd only allow non-frontosa females or fry/small juvies that you would normally imagine he'd eat-- he was just an odd individual.