Firstly, as others have said, congratulations on what will be the start of a life long addiction. 
Most rescues work on the rule of thumb that approximately 20% of greys can be cat trainable/friendly. Empty was unusual and lucky in rehoming a grey taht had lived in a working yard with cats around - that's not the norm.
What generally happens is that hounds are assessed for general prey drive - anything with a high prey drive (2 of our 3) wouldn't even be tested as you know it would only go one way. For low prey drive dogs, rescues will try to do meetings, and in some lucky cases can foster with homes that have cats.
Most rescues will have a waiting list for cat friendly greys, but they do exist, you just have to wait a little bit longer for them.
I'd also add that Empty's experience was also slightly unusual in that her grey had literally come STRAIGHT from a training yard. Normally, they come into the care of the rescue who will have them for a bit. Females will often have had season suppressing drugs while racing, so sometimes you have to get that sorted and them neutered asap, males are always neutered before rehoming and most rescues will try to do as much assessment as possible. The gold standard is for the grey to be in a foster home - in this way, the first introductions to stairs, washing machines, comfy sofas etc can be got underway and a more detailed assessment of temperament made. What we also find is that dogs straight from a working environment simply aren't used to being a pet so will appear to be quite closed down in terms of interaction/play and so on. One of the most magical things is watching the greyhound learn that actually these strange flat faced people can be fun to be with, and do loads of good things - they blossom into playful affectionate love sponges (though still very lazy).
Have a chat with your local greyhound rescue to talk through the adoption process and to discuss the availability of cat/small friendlies. If you need any help in finding a rescue or have any questions, just let me know - more than happy to help. Am delighted to find someone to bore/brainwash on the subject of hounds! 
Oh, and did i mention the collar porn?