it's possible.
i had to consolidate tanks for a few days resulting in little other option than a medium (2, 2.5") maroon female being in a 50g tank with a pair of ocellaris...first couple of days, they played well together...day 3 the maroon shredded both ocs fins, and chased the female to a point that it jumped out of the tank. the maroon quickly moved in on the male and began doing the 'dominant/submissive' dance with him.
in other words, it's not so much a problem of one of the black clowns pairing off with the maroon...it's more a problem of the maroon eliminating one (or both) of your black clowns. if i'm understanding you right, the maroon is smaller than your blacks, so while i wouldn't rule out aggression, it probably won't be too horrific right off. eventually, that maroon is going get bigger...and more territorial.
anything is possible, but i'd say the odds of calamity are high, especially if these three fish are in a 29g.