I agree with all the members who stated: every Greyhound is different. Mine prefers playing with small dogs, and although she is quite brusque and often too quick for the other dog to keep up with both for chase and contact play, she has never injured another dog - although she has unintentionally frightened quite a few of her playmates! At the moment, one of her favorite friends is a small Jack Russel male.
I think upon 'testing' a Greyhound with a smaller dog (with a muzzle for safety) it often becomes obvious if the Greyhound in question is able to adapt to a smaller, more fragile playmate or will eventually begin to instinctually treat the smaller dog as prey. It can be difficult to get the feel of your Greyhound's attitude in one play session though - my Greyhound will often bark and growl a lot while playing, this is simply her way of expressing herself. However, I have seen another Greyhound that gave a lot of attitude in her body language typical of play (play-bow, tail up, etc) but began chasing a smaller dog in a manner that quickly lost all semblance of play and became very threatening for the smaller dog (and owner!).
Do not take Greyhound out of your options as a potential companion - just find one that will click with Casey, not an impossible feat!