Chins can fight with each other for any number of reasons...ONE is pregnancy...but there are SO many reasons, and no reason at all, that chins can fight and to the death. It's not worth it IF they're already showing this issue.
That said, as far as a collar goes...collars are for run breeding. Special cages, special collars. They're not for use in regular cages, they don't prevent a female from attacking a male (or a male from attacking a female). What they DO prevent is a female from getting out of her cage. The collars keep the female in, the male stays uncollared and can get out of the cage. Keeps the male safer if he wants to escape but does NOT keep the female safe from an aggressive male. Does NOT keep the male safe if he cannot escape in time or in a panic.
That all said, I would say if your petstore is willing to give out the breeders names...that's one thing. You can get in touch with the breeder, get more information and maybe make a more informed breeding decision. However, this is not likely to happen. Petstores don't care about that sort of thing and worry that if they give the info out you'll go direct to the breeder for purchasing future animals and will likely get a better deal than they can offer...decreasing the petstores sales and profits.