I have owned over one hundred dogs, most all spayed or neutered, in my life time and have found they pick buddies like people do. Usually I had 4 to 6 at a time and bonds were formed not on sex or size but for reasons I have never entirely figured out. My working GSP's best friend was a shih tzu and they would play together by the hours excluding their other canine companions. I had a black mixed breed hound that when rest time came would go to a neighbors porch and bask in the sun all afternoon curled up with the neighbors black cat. We had our own cats then but they were not his buds. The only common denominator seemed to be their individual intellect and activity level.