"Small" wheels are just like everything else we do/use in knifemaking..... Give-n-Take. By that I mean that there are different advantages and disadvantages to each type and size. Personally, I am not a fan of any of the small wheel systems that have the bearings mounted "outboard" particularly on wheel diameters that are smaller then the OD of the bearings..... often times the bearings will get in the way, and limit what you can do with a given size wheel.
I favor small wheels that have internal bearings (the best example I can offer is to take a look at Bader's small wheel attachment)..... these type wheels/systems do not get in your way when grinding, allowing for far more versatility.
As far as sizes go, Although it's nice to have every size of small wheel, just in case you need it, over time I found it was a terrible waste of money. At one time I had just about every size small wheel available. The majority of them sat around gathering dust. I consider "small" wheels to be those that are 1" of less in diameter, and my platen already has a 1 1/2" wheel installed on the top and a 3" at the bottom (
http://www.caffreyknives.net/grinder_platen.html ) which are sizes I use for different area of almost every knife I produce. The only small wheel/attachment that I keep/use is a 3/4", which is used for the area of guard/handle junction on knives with guards. Eventually, I gathered up every small wheel except my 3/4", and sold them off. Now I only use/keep 3/4" small wheels in the shop. If I need a smaller diameter then that, I find it much more user friendly to simply tear a belt down to size, and use to with a slack belt attachment.
Why are rubber wheels safer than all metal wheels?
Let's talk about this situation. While you see all metal small wheels being sold, I have learned that they are far more hazardous to use them rubber coated ones. Why? The main reason is that because there is NO "give" in a metal wheel, The smaller the diameter used/run, the more likely you are for simply "peeling" the belt joint as it goes over a very tight radius...... the heavier the belt backing, the more often/more likely it is that the belt joint will pop when it goes over the radius of the wheel. This situation also happens readily on any belt with a mylar backing, such as micron belts. These same things CAN happen with rubber coated wheels, but it's far less common then with straight metal small wheels. Rubber covered wheels simply are more forgiving, and lessen the chances of getting smacked or sliced across the face when a belt joint fails.
If using small wheels, I feel that variable speed on your grinder is a necessity. As has been mentioned, small wheels will simply not tolerate high speeds. Either the bearings will burn out, or the rubber covering will simply disintegrate. For me, I've found that using small wheels with internal bearings is the way to go. Keeping the speed down, and only running them for a given amount of time (no more then 10 mins before allowing them to cool down).
That's my input on small wheels. As I said in the beginning, there are "give-n-takes" to just about anything we use or do in knifemaking. It's up to each individual to decide if the advantages out weight the disadvantages. Just remember...because you see it out there for sale, doesn't mean that the seller knows the ins and outs of it. I'd much rather give you my experiences, and tell you to stay away from all metal small wheels, then to hear/read that someone had a serious shop accident due using them.