As long as you're prepared to house them separately if necessary, I don't see a problem (and you will need to house them separately to begin with for the 30-day quarantine period - they will need to kept in separate rooms and you'll need to wash between any contact). There's no real "formula" for picking chins that get along - age may or may not make a difference, since every chin is different. I'm not sure if it applies to girls as much as boys, but I recall hearing that they may need to rework their dominance issues as/after they go through puberty, which may be a factor if the new girl is very young.
There are a few ways to do introductions. The typical method is a gradual one, where you start by putting the cages in the same room and gradually move them closer to each other over the period of a couple weeks. Once they're settled there, you can try cage switching - having them live in each others cage for a few days at a time, which will get them used to the scents. You can then try playtime together - if that goes well for a week or two, they're likely ready to be in a cage together for short (supervised) periods of time - which you can then work up the time. Other methods include "smooshing," in which you place them in a very small cage/carrier for a short period of time (they'll be so close that they cannot really fight and end up bonding instead - this can be risky though), as well as cage-in-cage, where you place one (usually the dominant one) in a small cage/carrier with food/water in the larger cage with the other chin, and leave overnight.
Good luck!