Cat mating behavior (? about yet another cat)

A girl that lives in our building found a cat outside her apartment door and took it in but her roommate wouldn't let it stay, so of course we took the cat in.
She is very thin, probably only 5 months old, and in heat. She has a small scab under her neck. I have heard that males will latch onto the scruff, so I thought this may have been the cause of the scab. However the scab is underneath the chin/neck, and I also read that cats will no longer stay in heat once they find a male cat to mate with.

We're getting the cat to the vet tomorrow for shots and a checkup, but I thought I'd see if anyone may be able to fill me in on whether or not the information I've read is accurate. I'm worried I may have another cat that is pregnant (or has mated with a male recently) but I'm not sure if everything I read was correct.

Also, this cat seems very aggressive. I'm keeping her in the bathroom for now, but I understand that cats are more aggressive when in heat.
She loves rubbing her head against me, but she doesn't seem to like being held at all. Is it likely that a cat that has been outside for it's entire life (even if only 5 or 6 months old) will never get along with other pets?
They are curious about her, but haven't really shown any aggression. She growls and hisses when she hears them on the other side of the door, and she loves me but doesn't like being picked up, which makes me wonder if she is feral and will not be capable of being a housecat. Especially with a small dog and 2 other cats. The one has never been around a cat that tried to hiss at it because I raised it from 1 week old and she probably wouldn't even try to defend herself if another animal got too rough with her. She has spent a lot of time outside the bathroom door trying to talk to the new cat, making her squeek/beeping noises like when she talks to me. The new cat hisses and growls, but Teddy just keeps trying to scratch through the door to meet her and keeps trying to talk to her. I'm also very worried that this new cat will not understand why Teddy is so intent on coming to her to meet her, as Teddy was raised by me and has no fear of anything (she has no idea how scared a new cat must be in our house, and just keeps trying to scratch through the door to meet her).

First off make sure you continue to keep her seperated. She could be ill, FIV or FeLV positive, or the scab on her neck might be fles or ringworm and she most likely has worms too.

Second, it is hard to say if she is pregnant. It all depend on when she might have mated. If it was just recent then her heat might not have stopped yet. I would take her in to the vet and have her spayed. You don't want such a very young and possibly ill cat having babies, that would bewrong and just awful. If you just want her spayed and don't want to know if she is pregnant then tell the vet there is a possibility but you don't want to know. It really is not a good idea for a cat this young to have kittens, she is still such a kitten herself.

She definately needs to stay in. Cat I know of have always been way more loving when they were in heat.

The vet said she tested negative for any diseases and is very clean and healthy (ears were clean, teeth, etc). He also said the scab is probably a bite from another cat.
We forgot to have a sample tested to see if she has worms, but she got a pill to take anyways. For now she's definitely staying in the bathroom away from our other pets. Especially since I've read that they may spray on things when they're in heat.
As soon as she's no longer in heat we'll get her fixed. She is very friendly but very skinny, so it will be interesting to see how she adapts and gets along with our other pets.
Apparently the vet said she'll be more submissive when in heat to our other cats but I heard they're more aggressive when in heat. Does anyone have any experience with this? I expected her to be more aggressive (not less) because she's in heat. She's definitely the one doing most of the hissing and growling when my other cats go near the bathroom door. She immediately hisses and growls a lot when she hears/smells them.

I think she is hissing and growling because they are new strange cats and she is in a strange place. Cats in heat are generally more affectionate. They want all the attention they can get and usually get annoying about it. It is males that get aggressive, but it is all the time when they are left intact. They are also more likely to spray then females but i have known some females to spray too.

I'd say the aggressive behavior is more of being in a strange place with strange cats/dogs/humans; rather then being in heat. Cats go in and out of heat quickly so hopefully your vet can schedule on a short term notice!

You have to keep her confined till then. She may or may not be pregnant - if she's mated, its more then likely she IS pregnant and at 5/6 months old, it will be tough on her development. If she is pregnant, will your vet still spay her? I'd try to get her in sooner then later, in case she's pregnant. You really don't want her to be having kittens at this young age.