Wonderful! Congratulations on your greyhound. I have a 3 year old grey named Piper, a Cavalier King Charles Spaniel (toy breed) called Winston and, ruling the roost, an Oriental Shorthair cat named Yoda.
The adoption group you are working with should be able to give you tips on the introduction, but this is what I remember...
Piper had been tested cat safe but I had one failed adoption with a "cat safe" grey that found Yoda's high level of activity too attractive, so I took no chances. Your grey should have a basket muzzel when he/she comes to you. Use that and a good strong grip on the leash for your introductions. Crate/leash the grey or confine the cat so they are not together unsupervised until you get real confidence that the cat is safe. Remember that even a grey that is great with cats indoors may be a threat to the same cats outdoors. Any interest, focused staring or play attempts by the grey should be met with a firm "NO KITTY". Clip the cats claws to protect fragile greyhound skin. During the first few days of adjustment the crate can be a real benefit to the grey and a way for the cats to have some safe freedom and an opportunity for you to gauge your dog's interest in the cats. Even after everyone is settled in nicely, make sure the cat has access to safe retreats where the dog cannot follow.
Piper is fine with the cat and the smaller dog and chances are that your new grey will be great too. However, if you find that you are concerned about the dog's reaction to your particular cats do not hesitate nor feel any guilt about returning one greyhound and trying the adoption process with another. Cats, greys, other dogs, and people ALL need to be happy and well-matched for the adoption to be a success and it is worth the effort to find the right dog for your home and family. If you have not done so, visit
www.greytalk.com. This is a BB for greyhound owners. It has excellent search capability and it you search you will find loads of info and opinion on greys and cats that should help. You will also find comforting pictures of greyhounds and their feline friends. It is also easy and safe to join and post with questions specific to your situation.
Best wishes and enjoy your wonderful new greyhound!