I think I have done it all. Had same breed, same gender. Different breeds, same gender. Same breed, different gender. Different breed, different gender.
Both when it comes to dogs and horses, and cats for that matter.
With giant breeds, the plus I can see that they play a bit different and it is easier for them to play with someone their own size. Other than that I cant really see that same breed should be of an advantage.
Usually different gender is easier, especially if you alter. If not well you have the heat/pregnancy issue. Mind you Zeus is an altered male and he and Hera were fighting badly as most of you know, so different gender is by no means a guarantee that they will get along. I have also had same sex dogs and horses that get along super well. So I think it depends on the individual.
Marit