Also, if it is a valid tag should it be six-sigma, 6-sigma or what it is now 6sigma?
I wondered the same thing, re: the name, the other day, and I ran a query at data.stackexchange.com to see if any other sites were using it. That only got me so far (no one was) because I think it only queries non-Beta sites (?).
Anyway, I know that PM SE has a 6-sigma tag. It would be nice that if we used it, it was used consistently across related SEs. Personally, I would have gone for six-sigma in both cases, but I don't have the rep to make those changes.
As to whether or not it is too localized, I don't think so -- but I could be easily persuaded another way.