It's a relatively fast-growing species, I think 4 inch pots would hold them for a couple years - maybe put that smallest one in the center in a 3 inch pot. They're not a shallow-rooted species, they like full-depth pots.
The thing about too-large pots is that if you put a cactus with a small root system in a large pot, roots aren't going to reach down into the bottom of that pot for a long time. The soil down there will remain saturated for prolonged periods after watering, and may go sour. It's less likely to be a problem with very free-draining media, or with fast-growing cacti, but it's still best not to use pots that are greatly larger than the existing root ball.
Cleaning the peat off the roots isn't easy, but don't worry too much about losing some roots in the process. Plants maintain a balance between the size of their tops and the size of their root systems, so lost roots will be quickly replaced. The spines on those are not very nasty, fortunately, so I don't think you'll suffer too much damage
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DaveW has good points there. A little detergent will work as a wetting agent and make the peat absorb water faster. You probably will need to weight the pot down to keep it submerged, or hold it down for 15 minutes or so.
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