Our vet spayed Jersey a month early due to her "sexually advanced" brother who belonged to a friend of ours and was over quite often! But I'm not sure about males.
As for what sex your next dog should be, can you tell if your dog will try to be alpha? Is he around other dogs so you could get an idea? We were not aware when we got Jersey that 2 of the same sex was not a good idea, but with Serene's past, we knew she needed to remain the alpha dog. Jersey was the runt of her litter, and you could just tell she was a natural follower, so she was perfect for our family! Even when others are over with their dogs, Jersey will just let them boss her around, but Serene remains "top dog"! Hubby says our next dog has to be male to balance out the male/female ratio in the house (he feels alittle outnumbered these days with just him and Nike for boys!!)