How these fit into your rack
I bought these to supplement my set of anodized aluminum stoppers (down to 3), thinking that they would protect MUCH smaller cracks than my aluminum stoppers would. This has proven to be somewhat of a misconception, although the Micro stoppers fit shallower placements better than the aluminum ones, the #3 Micro fits cracks of a very similar width as the #3 alu. stopper,(see photo) likewise for the other sizes.
So why would you buy these (for free climbing) if you already have the aluminum set? First off, I have found that Micros seat better than alu in the same placements. This means that they are less likely to get wiggled out as you pass by, which is great, because if you're placing gear this small, its likely your only option for pro, so it NEEDS to stick. Secondly, because of the narrower head, Micros fit in shallower thin cracks that Alu stoppers would be unstable in. Third, Because of the lack of a cable crimp (the cables ends are permanently fixed in the stopper head), Micros are lighter and rack cleaner than equivalent Alu Stoppers.
For free climbing, these are really only necessary on a minority of routes, most of them 5.10 and harder. That said, these hardly take up any space or weight on your rack, and when you place one of these, there's a good chance that its the only thing that fits.
For free climbing, I'd recommend sizes 3,4, and 5. The 6 is hardly micro, and 1 and 2 probably wouldn't hold a fall. Also, 3+4+5 combined weigh about as much as a Neutrino, so they won't inconvenience you if yo don't use them on a pitch, but you will be STOKED if its all that's available and you brought them along.