Since you say you already know all about breeding them you should know they should be full grown before breeding to make sure they grow up to the standard you are planing to breed for. You can introduce them in cages next to each other anytime though. You wont know actual adult size and coat until they are at least a year, not to mention it's not very nice to the chin to be still growing herself and be growing kits. I'm also assuming you got the chins from a breeder so you know the genetic line at least a few generations and that it's clear of all issues. It's best to have an idea of what you are trying to breed for or improve so you can select a female.
I know you say you know all about breeding being a CVT (I'm guessing certified vet tech?). But if you just know about breeding other rodents, or cats and dogs, then please take some time to read though chinchilla breeding. A lot can go wrong and they are not like any other animal when it comes to breeding. Just because you may have gotten lucky with your previous chins and nothing bad happened does not mean you will always get lucky.
Just keep in mind that not all chins like all other chins, you can pick out the perfect mate for him and they may not ever like each other. In terms of getting along though, age doesn't matter as much as temperament. You can't have dominate/dominate and if you have dominate/submissive then you need to keep an eye on them, and make sure the submissive one doesn't get too bullied. So basically find a female with a complimentary personality and that fits with your breeding goal.