I think Mathis will be used mostly as a return man his rookie year with some spot duty at WR in Bradford's role as a "go deep" WR. Next year, after he has had a chance to get comfortable with the offense and the speed of the NFL game he will probably take Bradford's roster spot and compete for a starting job with Gaffney and Armstrong.
Pettway will probably be on the practice squad this year. If he does make the team, he will be mostly used on special teams and would only see time at LB in emergency situations.