If you are trying to establish a breed, then you need to be concerned about inbreeding. Not in the immediate future, but eventually. I would recommend getting all unrelated birds, at least 2 males and 4 females, or maybe even 3 & 3. The more birds you can get, the better, but I think that that is probably a good number to avoid inbreeding problems down the road. It doesn't help to have a larger number of birds who are already related.