Of course that would mean 4 Medics/Engies/Assaults/Fire Support + 3 Snipers + 3 Hybrids, and only 2 new mercs would fill the perfect 24 merc number.
Anyways, there's constant label on how Guardian doesn't heal, Javelin doesn't give much ammo and Phantom doesn't spot. Thus not fitting their respective groups and being pale, compared to other merc.
Well, maybe it's time to make a special group? Heck, I'd be fine with that, no need to restrict your mercs to just 5 groups.
Like TF2, buried under gambling addicts and whales, forgetting the core gameplay.