Will waiting to spay my Mini Aussie Shepherd until after her first heat affect health/behavior later in life?

I have another female dog in the house and she was spayed very young; we got her when she was 12 weeks old and she had already been fixed. She seems to be in good health but she is a mutt and I hear that it is different when the spaying is of a pure bred dog. I know of multiple dogs who have not been spayed and... show more I have another female dog in the house and she was spayed very young; we got her when she was 12 weeks old and she had already been fixed. She seems to be in good health but she is a mutt and I hear that it is different when the spaying is of a pure bred dog. I know of multiple dogs who have not been spayed and they are in perfect health. The only problems with the one I know best, a border collie mix, are that she is rather aggressive towards other dogs all the time, especially my Aussie pup, and not only when in estrous. When she is in heat, she attaches herself to the 2 men who live in the house and if the men don't constantly discourage her protectiveness she will try to keep me, and everyone else, from getting anywhere close to them. Are these behaviors common with intact females and something I should worry about?
I am also considering not getting her fixed at all.
I would really love any advice I can get, especially from vets or breeders.
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