On the subject of small wheels for Roller Claws and the like, may I suggest looking at the Bane Bots wheels? You can get them with a 3/8" keyed, 1/2" keyed, 1/2" hex, or 3/4" hex center bore, they're relatively cheap, and from our experience more than strong enough for small mechanisms such as a roller claw.
The only reason I would lean towards these over say, a vex wheel, is that they come in some smaller diameters and are much easier to drive than a vex wheel, unless of course you were to put some sort of hub onto the vex wheel.
http://banebots.com/c/WHB-WM