You should do what you want. This is your horse and your decision. But if it were me, I wouldn't. Only because the friesian has such distinct and bold features that really work best with other friesians. Many of these beautiful baroque breeds have been carefully bred for hundreds of years to perfection to represent the breed standard. These are noble, elegant creatures used to represent royalty and as such I think they should remain in that class. Of course, they are used for all sorts of purposes now but they remain dignified and noble in the way that only a friesian can. It would be a shame to dilute that with a stock horse. A quarter horse is a wonderful breed too and dignified in their own way. But to me (and this is only my opinion) they are just too drastically different. What is your intended use for the baby? Dressage? Jumping? Trails? Ask yourself this before you decide on a stallion. The cross should never be about looks but about your intended use for the offspring. I have ridden many friesians and they are wonderful but some disciplines are harder for them than others.