The Fijian team is awesome already, I don't understand where he'd play for them considering they made it to the olympics and are the number 1 team in the world. You see the Fijians play and its unbelievable, nothing like a rugby league or even Wallabies team plays.
Let's wait to see what happens. Ben Ryan will make the decision, he took over an under-performing squad (which was, I suspect, riven with tribal/political enmities), and has brought them back together as a world force.
From what little I have seen of Hayne, he does seem to have exceptional abilities. If he achieves the fitness levels, and subject to his acceptability amongst the playing group, he could add an X factor that brings the element of surprise. All these teams know each other pretty well, strengths and weaknesses, etc.
Nobody knows anything about Hayne in Sevens yet.