Congrats for rescuing a kitty!
Here's what you need to do - she needs to be separated in her own room initially anyway - health risks or not, and there are some.
She will be scared. Cats are territorial, and despite the fact that she'll probably come around pretty quickly, she'll need time to adjust. That happens best in a small territory, like a large bathroom or guest bedroom or something. And introducing cats always works best when they're separated for at least a few days if not a few weeks. Monkey needs time to adjust to the idea that there's a cat with which she's going to need to share her territory, so letting them smell each other's scent under the door is the best way to go anyway.
Your new little kitty won't understand what's happening to her, and never having been inside a house before, everything is going to smell and sound really scary to her at first. MAKE SURE THERE IS NO WAY FOR HER TO ESCAPE THE ROOM. No pipe holes, no holes near lights, if it's a bathroom, no holes in the wall for plumbing. You cannot take her wanting to run and hide personally. It is instinct that will drive her at first.
But DO make sure she has several places to hide within the room. Under a bed is great - a box on its side is great. Under a table that has a cloth over it is great. Stuff like that.
If you've got a CD player or a radio, leaving classical music on for her is good. I'd put a nightlite in the room.
Knocking on the door gently before you enter and telling her you're coming in helps.
Keeping a schedule helps - especially when it comes to making sure her water bowl is cleaned, her food dish is full, and cleaning her litter box.
Until you can get her to the vet, do not wear shoes/sandals/slippers into the room that you use anywhere else in the house. Best thing to do is to keep a pair of flip-flop thingies just inside the door of her room - take of your shoes and socks off outside the door, and put those on. Leave them behind, and immediately go wash your hands.
Best to keep her separated for two weeks (see how it goes after that). But if she's got a virus (like a cold or something) it'll show up by then.
In the meantime, give Monkey extra play time to work of the stress.
And spend as much time in new little kitty's room as you can - ignoring her. It sounds counter-intuitive, but just letting her get used to the new sights, smells, sounds - and you (and or kids and hubby) is so important. Read out loud, sing, work on a laptop, fold laundry, sew, knit, watch TV - whatever. She may come over and headbump you - then pet her. But TRY not to force yourself on her (it can be hard!
) and let her take the lead. It really works best in the long run that way.
Obviously put some toys out for her (if they're interactive wand toys, do NOT leave them out - ferals tend to chew the string and it can get wrapped around their intestines). She doesn't really know what play is yet, but she'll figure it out with time.
You may want to consider purchasing Feliway. It is a synthetic hormone that mimics the "friendly" markers in cats' cheeks. Spray it around the house and in her room - it will help both Monkey and new kitty to be less stressed.
Also, you may have to use potting soil instead of litter at first - or even just a layer of potting soil over the litter. I also HIGHLY recommend putting two litter boxes out for her to use - ferals are not used to going pee and poopy in the same spot.
I would also consider procuring Frontline or Advantage and applying it to her ASAP (the safest, best way to kill fleas and ticks and quickly). When you take her to the vet, if there's someone else in the house, buy a flea collar, chop it up into three or four pieces, and put it in the vacuum cleaner bag or cylinder, and vacuum the room while she's at the vet, just to be on the safe side. If you're both taking her to the vet, then leave her in the crate outside her room while you give it a quick vacuum.
She will need to be de-wormed. There is now a topical for this, called ProFender. Make sure the vet applies it when she goes in (maybe talk to the vet about applying it just a few days after the Frontline application - I don't know about this yet). HOWEVER - even if it's OK to apply the ProFender a couple of days after the Frontline, you need to be aware that it (like any de-wormer) only kills the adult worms. The gestation period is 2 - 3 weeks, so a second application of whatever is used - if it's an oral med, the same thing applies - she will need at least one more dose three weeks after the initial dose.
As to making introductions? I'd bring Monkey into the room for 10 - 15 minutes at a time, supervised initially. Of course, that depends upon her reaction to just having another cat in the house. If there's hissing and spitting from Monkey at under the door (not uncommon), then I'd immediately start swapping scents. Take a towel and rub new little kitty all over with it. Put this "new kitty" scented rag under Monkey's food dish. Take another one, and rub new kitty all over with it - and put treats out twice a day for Monkey on that one. Do the same thing for new little kitty with Monkey - rub Monkey all over, and put that "Monkey" scented wash cloth under new little kitty's food dish. Take a second, do the same thing, and put treats down for her on it (don't have to wait or watch her eat them). This will help them come to associate each other with "good things."
Also, if Monkey is just curious about the new scent and sounds through the door, simply praise her. And on the first intro, if there's no hissing, again, praise them a lot for being nice kitties. But be ready to wisk Monkey out of the room. Maybe want to have an empty can with coins in it ready to shake - that's only if they get into a fight before you can stop it. Do not EVER try to separate fighting kitties with your hands. The loud noise of the coins in the can will startle them out of it (if it even gets there, which doesn't happen too often, but best if prepared).
Just remember - take it slow, be guided by what you sense from both Monkey and new little kitty, give it at least two weeks on keeping new little kitty separated for Monkey's health, make sure you follow-up on the dewormer and the distemper - both are three weeks after the initial ones - and remember to praise them for good behavior. It's just as important as telling them "no" for behavior you don't want.
And once you open that door - leave her room set up for a while. It will continue to be her "safe" space to which she can retreat when scared.
And with the two kitties, I would keep at least three litter boxes out (rule of thumb is the number of litter boxes you should have is the number of kitties you have plus one.
And remember - we love pics and are here to answer questions along the way!
...and now that I've written all of this, she'll be a total love bug, comfortable within two days, and Monkey and new little kitty will be chomping at the bit to meet each other after two weeks and will be best of friends within two days!
All of this is because while it sometimes does go like that - with a slightly older kitty it usually takes a little while, and it's best to just turn off your clock and your expectations.
She WILL come around. And we've found it easiest to introduce new kitties to males - our females all get jealous, and it takes a while for them to accept another female into their territory - let alone into their hearts.
for what you're doing!