i think you should just pick the team on merit. with all due respect, there are 7s players on the circuit for wales, england & scotland. it would be a disrespect to 7s to suggest that a player could just go from 15s to 7s. the physical requirements are different.
without knowing the fine details of eligibility, i would imagine that any 15s players with a chance of playing would switch to playing some 7s events 2-3 years before Rio 2016. i dont think we will see 15s players just turning up and slotting into a squad which will have 3 national teams vying for it!