I do not recommend breeding a chinchilla until they are one year old. More important than age though is whether the female is big enough to safely deliver kits. I do not breed any female that is smaller than 600 grams, and prefer them to be 700 grams or more. You also need to check "the rule of thumb". If you feel just below your chinchillas anus, you will feel a ring of bone, the hole inside this ring should be at least as big as your thumb. If it's smaller than that, the female may not be able to safely push kits out.
Some females change personality during their pregnancy, some don't. It just depends on the animal.
It sounds like you are fairly new to chinchillas. Where did your chinchillas come from?
Do you have pedigrees for them?
Breeding is a HUGE decision and responsibility. Too many people jump into it before they know what they are getting into. Have you done research on genetics and what to look for in a quality cihnchilla? You do not want to breed inferior animals, as they can be prone to genetic diseases which can be passed on to their offspring. These illnesses, like malocclusion, are very painful and usually fatal.
You really need to put a lot of thought into your decision, and not just about whether or not you want to do it. Do you have the money for possible vet problems? I had a female who needed an emergency c-section. It cost me $1200, and she ended up not surviving after the fact, which brings me to the time aspect. If a female is unable or unwilling to nurse her kits, you will have to be able to handfeed them every two hours aroudn the clock for several weeks after their birth, and then every four hours for weeks after that. Do you have the time for this? Do you know how to hand feed a chinchilla kit? Do you know how to handle other possible complications, like breech birth, or if the kit gets stuck in the birth canal?
If you don't know the answers to any of these questions then you need to take some more time and research about breeding before you do it. It can be a lot of fun, but it can also be heartbreaking, stressful, and expensive.