Well, it seems you're doing everything right. Some cats are just more set in their ways - and some cats just don't like other cats that much.
In our experience (we have seven cats), the females are more jealous and take longer to accept new kitties. If we waited for one of our girls, Spooky, to stop hissing before just letting newbies have free roam, they'd be separated for six months.
I don't know what your goals are in terms of integrating the new kitties. At least they have each other for play - so I wouldn't worry too much about that. If they try to play with the older kitties, they'll get bopped or tackled, and they'll learn to give that up (hopefully sooner rather than later).
What I would do, at this point, is just focus on minimizing the stress to your existing kitties. It does not sound to me like you need to get to a place where your female is no longer hissing. If she's anything like ours - your kittens will be separated for a long, long time.
If you haven't already purchased Feliway spray, that's often VERY helpful in a multicat household. Some people recommend the diffusers - that's fine, but they're expensive, and if the cats don't kind of concentrate into one or two rooms in your house, it's kind of pointless.
I would also consider giving Bach's Flower Essences a try - specifically Rescue Remedy. You can add 3 - 5 drops in their water dish(es) every day when you clean it (them) and put fresh water in. You can also dab a little behind the older kitties' ears, at the base of the neck, at the base of the tail, and under the chin (this one wafts up into their nose). It may help calm her stress about having new cats invade her territory.
Um - the boys are 6 months old. Are they neutered yet? That'll be important, because they'll be less "territory" oriented.
FYI - for your future reference, you may want to start thinking about vertical space now. Having as much vertical space as possible in a multicat household is REALLY important. That also really helps diffuse the alpha-status and pecking-order issue that arises when there's more than just a couple of cats.
At some point you just have to go for it. You can try the supervised intros and see how it goes.
How long have the kitties been with you at this point? It's so hard to tell - some people get anxious after just a couple of days, and some people give it a couple of weeks before worrying.
If you really want to go whole hog on it though - whatever room is their separation room - replace the door with a screen door, and give that a week, while continuing the scent swapping, and using the Feliway and Resce Remedy.
Another thing you can try to help perk up your upset female is harp music. Classical harp music. This was designed specifically for stressed kitties - http://www.musicmypet.com
- but really any classical harp music works (at least to ease stress. Doesn't make cats like each other!
One last thing.... I don't know what kind of scent swapping you're doing. For some cats, it's enough to put a "new kitty scented rag" in the bed of the existing cat. But we recommend it be placed under the food dish of the existing cat. And we also recommend that play sessions for the existing kitty - 10 or 15 minutes every day - end in a treat session where the treats are placed on a "new kitty" scented rag. It's not just about getting them used to the scent of the new cat - it's about getting them to associate that scent with good things, with food, with fun - and to think it's a total party having new kitty around.
You should play it by ear with all of this in mind. In the end, there will be hissing and growling, and probably some batting and scruffing. This is very normal - the "pecking order" and dominance need to be established. The idea of slow introductions is not to eliminate the need for that to happen - it is just to reduce the intensity with which it happens. You just need to be around to make sure no one is actually getting hurt (and it can sure sound like it some times!
) - but it is important, once you get to that point - to let them get their relationships established.
BTW - don't know if you live in a house or what, but stuff like this is easy to do if someone in the household or family is handy. You really just need brackets for shelves and cheap wood shelving. Carpet, of course can be stapled or glued on - but how cool is stuff like this? http://www.katwallks.com/customerphotos.htm