Good With Dogs:⬛⬛⬜⬜⬜⬜⬜
The American Bulldog is a very powerful and intense breed of dog. They need a TON of socialization as a young dog to ensure that they do not grow up with aggressive tendencies towards other dogs. Also because the breed has a fairly high prey drive, small dogs, children, and animals should always be supervised and the dog needs to be kept under control when around them. Since your dog is six years old and does not like dogs, the best way is to contact a Positive Reinforcement trainer and have him/her show you some techniques to help you work with his sociability.
Any dog, at any age can go through training. The best thing to do, no matter the age of the dog is to set up a training class. Solidifying your bond with your new dog is critical and it will help you do move forward into her being a wonderfully obedient member of the family.
There is not 'guard' as you say. When a female goes into heat, she goes through three stages. The first stage is when a females vulva starts to swell and she will start to excrete blood. At this stage, she will not allow a male to mount her. The second stage is when the blood will turn to a pale pink and/or a clean mucus and that is when a female will allow a male to mount her. The third stage is when the vulva starts to shrink, the female can excrete a pale pink to mild red mucus again that is normal. She will not allow a male to mount her at the end of her cycle.
Size, temperament and hip problems would be the three main things for me. But getting your pup from a reputable, responsible and knowledgeable breeder is the best way to get a well-bred pup. The dog should grow to be in the standard and it's temperament should be playful and happy-go-lucky with people and dogs. No fear or shyness as that can lead to aggression problems. And then making sure that the sire and dam have had their health checks to ensure that your pup has the best start with life as possible.
If you were considering an American Bulldog, your greatest concern depends on the home you bring the pup into. A general concern for me and most who purchased pups from me (I have owned American Bulldogs since 1994) is temperament. Be sure that your pup is sound. If their really shy, scary, or is showing any other behavior which may indicate a lack of confidence ... this can be a problem in the future. Just keep in mind that like humans, dogs have their own personalities too. A well bred American Bulldog should be very sound.