Most people breeding Frizzles breed only Frizzle to Smooth. Two copies of the Frizzle gene will cause feathers in 50% of the offspring to be brittle so they break off, or very sparse to start with. There is also organ problems that can occur. I raise Sizzles a breed being developed from a Silkie/Frizzled Cochin Cross. I use the Frizzled hens with the smooth rooster. It is simply a preference. The little girl Sizzles are usually nicer looking in my opinion. When breeding the smooth roo to the curly girls, they do suffer a bit more than a normally feathered chicken from bare back. I use saddles at the beginning of the breeding season to protect the backs and I try to keep at least 4 hens to one rooster or I allow him only several hours a day with the girls.
Not sure how much more I can tell you. If you have any specific questions feel free to ask.