Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.
Yes and no. Assuming that the collets are made to the correct spec, ER8 / ER11 can compress 0.5mm smaller than nominal and from ER16 up can compress to 1mm smaller than nominal. By spec you should NEVER go larger. Again, this depends on a good bore, taper grind, correct kerf size, and counter bore in the collets. Even within this range, any runout, clamping force, or slip resistance is only spec’ed for nominal bore size.
I’m somewhat less bias here as at least currently we don’t really offer metric collets. It depends on what you are doing with them. One of the important specs for collets is runout (how much it shift the tool off the center axis of the spindle). If you are dealing with small cutters, chiploads, or cutting metal this can matter a lot more. The manufacturers that I know of that spec that actual collet YOU get and not just an average or AQL are us (PreciseBits), Rego-Fix (inventors or ER collets and the R in ER), Pioneer, Haimer, and Technik’s UP grade. There are different grades from most of those using differing measurement setups/standards.
If you want some more info here’s some other threads I’ve posted to related to collets and collet nuts that might be useful.
And the one that goes off the rails about torque.
Hope that’s useful. Let me know if there’s something I can help with.