Can you use snaps for the cloth over the bumpers instead of valcro or with valcro?
Many teams have added snaps to the back of their bumper system. They use parts available from fabric stores intended for snapping down covers for trailers and tarps.
So snaps can be used, even though its not anywhere in the 2013 Manual?
Is there a certain restriction on the type of snaps and valcro alvaliable?
Only restrictions would be height. You shouldn’t have anything that keeps the bumpers more than 1/4" from the frame.
R29
BUMPERS must be supported by the structure/frame of the ROBOT (see Figure 4-6). To be considered supported:
B. the gap between the backing material and the frame must not be greater than ¼ in., and
You also want to make sure that the cloth can’t come off during the match
. . . and if you have a system where one color of cloth folds over the other color to use only one set of bumpers, be sure you completely cover the other color every time you change from one color to the other.
Also, that your corners aren’t ragged.
Dr. Bob
Chairman’s Award is not about building the robot. Every team builds a robot.
In other words you will be fine if you use snaps 
Make sure they won’t snag and rip off during a match either!
But really, you should be fine.