Fitzpatrick will be good at FS or CB. He's a good athlete, with solid fundamentals, and he's very smart; however, being an elite FS requires more than that. It requires high-end instincts - the ability to break on the ball/see the play quicker than is reasonably possible. That's what made Ed Reed so great, and that's what makes Earl Thomas so great. Neither were as fast as Fitzpatrick, and I don't think that either of them were particularly smarter. Their reaction times, though, were much, much quicker.
As I see it, Fitzpatrick has more upside at CB. He didn't do the agility drills coming out of college, but he posted a ridiculous 3.87 20-yard shuttle coming out of HS, and his agility is apparent. Match that with good speed and a solid frame, and you have a guy with some real upside at CB, which is the more expensive position anyway.
He's never going to be an Ed Reed, and you're never going to maximize his strengths at FS (again, the cheaper position). He can be good there, but you drafted him at 11 overall. Play him at CB where he has a chance to really thrive.
Another note, Ed Reed and Earl Thomas are RARE for a reason. Everyone wants some young player to be them, but you can't eat with hope.