I could easily see Brainy x Krypto. I even kinda like it. Personally I would like a Brainy x another Dog Star, but I dunno...some of them are a little small for her. ^^' Not that it really matters. Like Tusky/Mammoth; she's so tiny compared to him (at least when she's deflated!) Still, I could imagine it.
Actually, same thing with Ace and Rosie. I really like Rosie, but again she's so small against Ace. Maybe one of the smaller Dog Stars...
I was sort of looking at them physiologically. I can see how the personalities of Ace & Rosie might work together, and 'specially Tusky & Mammoth. There was one episode where Tusky referred to her as little, and she puffed up and said, "Who you callin' little?" XD