The "races" in the Quest of Queens campaign setting are not "races" in the way we might think of the social construction. Instead, each race is a fully divergent species, and while all races might be sexually compatible with one another due to their common ancestor, interbreeding among them is not possible (with some exceptions).
Strongmen & Humans
Strongmen and humans can interbreed, as it appears Strongmen may be an offshoot of the human race. The result of such offspring is a human with a shortened lifespan, whose physical attributes are enhanced by the supernatural strength afforded by the Strongmen. The children of humans and Strongmen are not identical clones of the parent. (When Strongmen have children, the offspring strongly resembles the mother or father.)
Humans & Yevim
There are legends of humans and Yevim intermingling and producing offspring, but none have ever been verified. Queen Madeleine II, for example, claims to be the descendant of a Queen of the High People.