Hi midyo
Spin: always a function of material and diameter. I have no trouble threading 7000-series Al or brass at 400 RPM on diameters up to 20 mm. To me, the limiting factors are sync distance and withdrawal speed. They might depend on your machine. Cut depth - not much, but always enough to actually 'cut'.
Yes, .062 is older than .066 or .067, but ...
.067 was only briefly available and was withdraw, afaik.
.066 is 'recommended' by the Artsoft web site, but users have had problems with it and most experienced users recommend dropping back to .062 . I suspect that whatever bug fixes they attempted with .066 made too many other things worse.
.063, .064, .065 - I don't think they were ever released.
Cheers
Roger