While it is both possible and legal to use a road rear hub there is one legallity which MUST be adhered to, Quick release axles are NOTto be used on front or rear. There is no legal requirement for the type of rear hub other than it MUST be a .FIXED gear. As far as I am aware there is no way of converting a cassette into a fixed gear, but the old screw on type hub is easily converted simply by screwing on a fixed gear instead of the cluster. As for the cranks, there is no issue there, other than if you are using a track frame it is possible that the inside spigot (for the small chainring) may make contact with the right side chain stay of the frame