Rats (Age, Introduction and Quarentine) XD

Question
Hello, (A lot of questions below!!!)

A couple of months ago i bought a baby rat from an exotic pet show, she was about 4 weeks old. I've had her for a couple of months and she is now around 4 1/2 to 5 months. She is currently housed with one of my other rats who's around 1 1/2 years. Her other cage mate (another older rat) has recectly died (last month) from a severe upper respitory infection.

-I'm wondering at what age would be ideal to introduce another rat to the pair. (Their both female, not spayed)

I was waiting until she was at least 6 months old before i went to buy more rats seeing as by then she should be full grown. (size wise.)

-Is 6 months of age a good time to introduce a new rat or should I wait longer?

I know if i do get a new rat then there will be at least 3 weeks to a month quarentine period.

-Is it okay to let the rats "play" together but not have then housed in the same cage for the 3 weeks?

In my house right now my room is the only room the rats are kept and allowed to free range (since i throughly rat proofed it XD)

-What would be an ideal room to place the quarentine cage?

I have both cats and dogs so there is really no room of the house where animals have not been at some point in time, plus i live in Florida so the garage is out since it can get VERY hot in thier and i would rather not have my ratties die before i even introduced them.

I know this is a lot of reading and question answering!! SORRY!!

I Look forward to your answer(s) as soon as possible!

Btw-
The older rat is very friendly and has already had a number of cage mates so i know she does fine with any new introduction. So I'm not to worried about her.

Answer
 When introducing new rats, the younger they are, the better.  So if you want to get more rats, do it as soon as you like.  With females it's pretty easy to introduce at any age, so it's unlikely you'll have any problems no matter how long you wait.

 When you have rats in quarantine there can be absolutely no contact between them, so no they can't play together.  You also have to wash your hands and possibly even change your clothes when moving from the quarantined cage to your existing rats.

 The best quarantine areas are completely different airspaces, such as a garage, basement, or even a friend or relative's house.  If you really can't do a separate airspace, another room will suffice, just be careful going from room to room, making sure you wash or change clothes to be sure no germs are getting from cage to cage.

 I hope that answers your questions, good luck with your rats!