Just an idea...maybe a mixture? Let some choose, if hilarity/angst/plot would benefit, and let other brave muns chuck their pups in for pot luck?

I suggest it simply because I have pups that pot luck could go badly wrong with - Javert, and Bruce Banner, for example. But Gene and Bruce Wayne would be fine with any random pairing. I'd happily throw one of them into the random pile, if I could choose for one of the others.

Your plot though, and I will follow whatever rules are laid down. :)