Hi and welcome to the forum, so lovely that you want to give this little one a good home.
Some cats never manage to live well with others but with care and patience you can give everyone the best possible chance of success. Think months not weeks, and at every step of the long process give it a week of each step of progress becoming normal and comfortable before taking the next small step.
First, is there any way you can make sure they never see each other accidentally? A screen maybe, or a barrier of some kind in her room to make sure there can be no accidental escape or visual contact at all, ever. If she should get out and there be a fight, it makes it so much harder to integrate them. Keep all experiences positive.
For the steps involved in detail there is plenty of guidance but the basic idea is -
1. Complete separation as you are doing now
2. Let them smell each other and get used to each other's scents, you can swap beds and toys between the gang and the newbie
Catnip, food and toys all help - if they can enjoy these things when the other cats are nearby, they begin to associate good things with each other's presence
3. Eventually when they are totally comfortable and can eat and play and relax either side of a closed door, you can start to let them see each other through the crack of the door only just open, with something weighty to stop it from flying open. If possible keep a barrier between them when you reach this stage so that they can see each other and sniff noses but not reach each other.
Try if possible to get her used to each of the three one at a time, three is a bit overwhelming.
4. Supervise them at all times and limit contact time, increasing it gradually as things go well.
- At the slightest sign of stress or discomfort, go back a step and give it even longer than the first time.
Remember a little hissing is not aggression, it is simply a cat saying back off you're too close and I am afraid.
The newbie knew there were cats there before she decided she wanted to live there, so that is positive. She is being defensive, not aggressive. Don't get me wrong a cat in defensive mode is a dangerous thing, but with patience and care she has her best chance.
Remember for a street cat danger is everywhere and although she is accepting your kindness she may have bad experiences with other cats chasing her away from her safe places and food. Maybe it's a good thing she feels this room is worth protecting.
If in the end despite everything she really cannot live with other cats, then at least you will be able to find her a good home where she can rule as an only cat and be happy, even if happy can't be with you.
Oh - and keep your anxiety and stress about the situation under control. Cats whether they are pampered pets or street strays are extremely sensitive to human emotion. If you are tense she will pick up on it and think there is something dangerous to be worried about. If you build trust with her and are calm and take her worries in your stride, she has the best chance of following your lead and trusting that if you're calm and confident then she can be too.