Not what I normally did and it's been years ago, but on occasion I had both severums and green terrors in hap/peacock tanks with no problems. The severums were ignored and ime green terrors can more than hold their own, in fact ime they'll boss or police a hap/peacock, tank, breaking up fights, etc.
That's just my experience-- it's one of those things you can't dogmatically say will work or won't work, but I wouldn't rule it out. A possible factor in my mind would be whether the Africans see the firemouth's shape as being similar enough to see him as a conspecific, which isn't much of a factor with sevs or GTs. It might also depend somewhat on the firemouth, whether he chooses to fit in or attempts to pick fights, etc.