"6mm switches" are a standard sort-of like "1 inch pipe", where you sort of look at the datahseet and ask yourself "WHICH part is supposed to me 6mm, exactly?" (it's actually a bit better. The "body" of such swicthes is usually actually 6mm.)
https://dznh3ojzb2azq.cloudfront.net/pr ... asheet.pdf(not the actual switch Adafruit sells, but a similar one conforming to the same standard.)
So, about 6mm*3.5mm body, mounting holes should be on 6.4mm centers...
You should realize that some of what Adafruit DOES is to pick low-priced vendors and manufacturers of various "jelly-bean" parts. and put them through "evaluation" like:
1) This looks and feels pretty good, and seems to do what we want it to.
2) The vendor sent us the same parts several times in a row. With predictable promptness.
3) We've sold them to customers and had few complaints.
This is a valuable service! But it does mean that there is not always an exact datasheet for each product