Two New Adpoted Cats Wont Get Along!

Cats are territorial, so it's not about them knowing each other or having liked each other in the past. The first cat we rescued we had in our home for 10 days before we rescued his brother. They fought like crazy - outside they got along just fine.

It is normally recommended that introductions between cats are made slowly, over time. That the newly adopted cat be confined to one room to give the kitty time to adjust to the new scents and sounds - the new environment in general, and to make that room his/her territory - their "safe" space - while giving time for the resident kitty to get used to the idea of another cat being in the home (being able to sense and smell a new cat).

Cats are not pack animals, and do not instinctively like each other.

If I read your post correctly, they've been in your home together for only a week or so (if that long?). This is a VERY short period of time to let them adjust to each other.

We live in a situation that does not allow us to separate new cats when being introduced. It takes one of our females a full six months before she stops swatting and hissing at the "newly" introduced kitty.

Given that they are both new to your home and your female didn't really have enough time to make your home "her" territory before you adopted the older male - at this point, just turn off your clock and give it time.

There are things you can do to help them though. Since its' the female having problems with the male, rub him all over with several wash cloths or clean rags. Put one of them under the food dish (if they free feed - if not, whenever you feed her, put down one of the rags under her food dish).

Give each of them "alone time" play sessions. When done, put treats down for her on a rag that smells like him. Put a rag that smells like him in her favorite place to sleep.

The idea here is to get her associating him with things she loves. Get some new toys, and when he comes into a room where she is, totally fuss all over her, give her a new toy. Make her think that having him around is a total party.

Cats learn well from positive reinforcement. Any time he comes into a room or is near her and she DOESN'T hiss or growl, praise her to high heaven!

Do you have much in the way of vertical space? Cats are very "3D" animals - having vertical space can help diffuse some of those "my space" issues.

...but most importantly, just give them time. This is a process that when done correctly can take weeks or months, depending upon the cats. Just because she's growly now doesn't mean they won't end up becoming best of friends. But they need your patience - and your understanding.

Just because they were in the same shelter at some point doesn't mean you can skip slow and proper introductions. Separate them and start over. You'll be working at this for a week or two, depending on what your female kitten can handle.

I've noticed that female cats are a bit more touchy to new additions, so her behavior doesn't seem odd. It's good that your male is kicked back enough not to get riled up over her hissing - it'll help a lot with introductions.
I'm going to both agree and disagree with this statement. While it takes time, and some cats will never get along, most cats really don't like being completely alone. It's pretty much the only thing we have going for us when taming ferals and trying to get cats to live together.
So you have an animal that is somewhat like a lion, socially, but with a lot looser bonds.

she may associate his shelter smell with bad feelings.

It will just take time. It can take a week a month, two months, more, for cats to get used to each other. I don't think it will take too much longer. It's probably his smell she objects to most.

Get a feliway plug in diffuser and try the vanilla trick. Put a dot of cooking vanilla (not perfume) on the nose and root of tail of both cats every day for a few days.

And, I realize space is limited but I strongly suggest you get another litter box.

Hurray for you for adopting from a shelter!

Welcome, and what are their names?

Yes, the Feliway is an excellent idea. The diffuser is expensive, but worth it if you can afford it. If not, the spray will help just as much as the diffuser would (you just have to respray every 3 days or so).

We live in an RV, so we never had the room to keep kitties separated for intros either. They'll be fine - with your help on getting the to associate each other with good things and Feliway will help reduce stress - the only other ingredient is time, and I'm sure they'll be fine!