er16 , er32, er40, and similar will go +/- .015 so 1/32 is fine for those. they are double angle parallel closing collets and have much smaller segments making them more flexible and allows them to conform to a smaller radius rather than having just 3 contact points., other double angle collets may go smaller by a similar amount but may or may not go bigger. i'm not really sure of too many other parallel closing collets and the recomended size range.
3c, 5c r8 and similar are designed for one size within just a few thou. for in between sizes you will need so get soft collets and bore them to fit. compressing single angle collets to fit smaller parts will distort the collet and you will have minimal holding area. it will also distort thin walled material.
i used to work in a factory that made pump seals and other products out of carbon materials similar to graphite but much much harder. we used collets to have full diameter work holding (though full contact jaws on a chuck would have been better, but who listens to me?...) every setup required cutting a soft collet to the part or stock diameter for concentricity and if you cut the diameter smaller than the piece that would fit in it, even if the collet would accept the part when open the parts would slip and you'd get tri-lobal parts that would appear round with a mic but would reveal the out of roundness when measured with an indicator on a v-block. (google or youtube "solids of constant width" to understand this). basically the goal was to exactly match the diameter or .001 under to account for compression when you cut the collet closed. metal is more forgiving than carbon though but i'd try to keep it within .005 when you can...
