If it was me,- i'd follow this line of Whippet X Greyhound, to hold onto the same "type" but drop the size a little. An old friend of mine had this "type" and it was very good indeed on Hare and Rabbit, i don't think he ever needed to work it to fox , so i don't know how it would have done there. I don't understand the thinking of putting Bull X Grey into it. Just my opinion though. Each to their own.