RE: Are Futaba and Hitec dual conversion rx crystals interchangeable?
Dual conversion crystals are cut to generate a signal from the local oscilator. Different recievers use different intermediate frequencies and have different charactaristics as far as impedence and the voltage they are set to operate at. Even though they may appear to work on the bench is no gaurantee they will work in flight and its fool hardy to take chances with your model and the lives of others to save 2 dollars.
As for legality, the fcc only cares about transmitters. From a liability stand point, if something happened and an investigation showed that you were going against accepted practice by mix and matching radio components your insurance and the AMA would probably (and rightly) leave you high and dry.