first; they are just polygons and textures, any resemblance to a real person is purely artificial. No character instance in CoX has any human rights, nor any rights period. You Own Your Characters (so long as they are not ripping off someone else's registered ownership of such a character. I.E. the NPC's are wholely owned by NCSoft/Cryptic Studios(in some cases)). No, they are not paid for their work.. I did tell you they have no human rights because they aren't real right? ok cool so when it comes to bashing or [Axe Melee. Beheader]+ing a "female" enemy of any type it's no different than a "male" or "huge" one. No it's not "too much to allow", Catgirl farms anyone?
second, I think the reasons there are not female members of certain enemy groups is entirely reliant on liklihood in the background of those groups.
In example:
Female Hellions are quickly offered as sacrifices in arcane rituals and turned into Girlfriend's From Hell. - so while we could maybe see some Female Hellions Initiates, they would likely have no attacks and just be being led to some altar somewhere. beating them up probably wouldn't be heroic, they need psychiatric therapy, and maybe a visit from M.A.G.I if anything. so it'd be more of a "there is a female member in the group, like the skull's girlfriend's who is just an NPC, but gets police TPed away when the mob is defeated."
- could make it that if there is no Hero intervention in that mob, or the mob is never defeated in that spawn cycle then the next spawn cycle will include a "Girlfriend from hell"
Female Outcasts- outcasts, let's put it bluntly, they are Super Jocks with, to be polite, kind of "Dick" attitudes and we are not talking "Tracey" here, they are often found making shady deals, harrassing eachother, shaking down random people for money with threats, or generally getting caught breaking into things. Unless they have a sports car and lots of cash, no girl would be caught dead with them, or maybe they secretly were caught dead by them. It's assumed some "Super Girls" may have wanted to join, were harrassed as "Super Candy" got butthurt and quickly found some other villain group to join, (probably the "knives of" brand)..
Female Freakshow - we have flambeaux she decided to join their ilk, but the whole implants and trading body parts of metal thing just wont go down with the ladies... if it weren't for the harassment I'd assume thats how female freakshow would look, kinda more like a goth female outcast with fishnet stockings.. maybe claws and some sort of armor with electric generating rods, but tasteful with electric decals.. probably tied up hair, or no armor so it doesn't get in the way of their hair. Keep in mind the freakshow are literally itching to fight and often fight amongst eachother for the fun of it. the first time a female freak got her arm lopped off, she'd probably, get shocked, quit, go to emergency and try to get it reattached and beg for a treatment to remove scars (probably ending up as a crey), and consider a career change. Men however.. I guess it's the testosterone. "you chopped off my arm, I'll implant a slasher in the stubb and chop off both of yours." style.
I could be wrong, but if I had to guess their reasons I would suspect these are fairly close.