What groupings of genders should be avoided?

Theoretically, a female and a male are the safest pairing. Behavior experts have said that spayed females can be the greatest risk, even more dangerous than 2 intact males. However, the vast majority of my dogs have been spayed females and I've never had interdog aggression between any of my dogs.

I would think that it depends more on each dog's temperament and upbringing. Dogs who have had exposure and good experiences with other dogs while growing up will be more likely to "play well with others."

It totally depends on the dogs. Trying to limit your clientele based on gender pairings would be really rough anyway. I've hosted dogs of both genders, intact or fixed, in all combinations and I've never had an issue. You can always ask owners at the M&G whether their dog has a gender preference. My friend's spayed female dog really prefers females over males; she gets along with males just fine, but she is usually less tolerant of any rudeness from them during play. Some dogs may have particular preferences, and over large sample sizes there are pairings that are more frequently problematic than others, but when you're dealing with a handful of individual dogs they are unlikely to follow the same pattern with any regularity. Basically, if you flip a coin 1000 times you should end up with about 500 heads and 500 tails, but if you only flip a coin 10 times, it's not weird to see 9/1 or 8/2.

Typically female to male best combo. Then male to male. Then female to female

However! There are alot more other things that influence whether they get along. Age is big factor. I find puppies less trained so less tolerated by older 7+ year old dogs. Also energy level matching is important. If your dog couch potato and you get springer spaniel, your dog may end up getting annoyed by the other being so hyper. Size is a factor. Some of the smaller ones can look like bunny rabbits especially to hunting dogs. Certain breeds are more standoffish or independent. (For ex akita, chow, pekingese) You also have to be vigilant when it comes to shy/fearful ones as dogs have a tendency to bully them. Other are more protective or exhibit guarding behaviors when it comes to food/toys.

And somethings it just comes down to that individual dog.