The Texans can keep more than 5 WRs if they want to. They probably won't but they aren't limited to 5.
I really don't see a front runner yet for the #5 position. Lewis has nice size (also dropped a sure TD), Starling has great speed, and Thurman has shown the ability to make difficult catches. Thomas finally made a play or two Saturday night, and Chris Taylor and Albert Johnson are bringing up the rear at the moment IMO.
I think the job is still JJ's to lose but if Duff shows sure hands and adequate yards per return, he could easily be the new return man which has its advantages because Duff can also be used on coverage teams or special DB assignments, etc., whereas JJ's value as anything but a return man is negligible.
Duff's chances just improved because it appeared that Jason Anderson was also going to be used on kickoff returns and now he's done for the year. The Texans won't say it but I think they really want Duff to succeed in order to get more bang out of the roster spot that JJ held last year.