I've always preferred to have multiples, mostly because that's what I like Most greys do appreciate having another grey around because, as said above, that's how they were raised. That said, many are perfectly happy to just visit other dogs and keep their people to themselves. None of my dogs have been closely bonded to each other, but they all seem to prefer having another dog around. I can see the difference when I lose one and for a short while just have one dog, but most of them would probably adjust to being only dogs.
I guess what I am saying is, get another dog if you want one, your grey will probably be happy either way. Fostering with intent might be a good way to test the waters
only downsides, more money for food and vet bills, more poop