"The dose makes the poison" -- a statement first made in the 1600s, I think, and that still holds today for most substances (some hormone-like organic small molecules violate the rule). Selenium is indeed considered a "micronutrient" -- many otherwise toxic elements are too -- but IMO you can't use that fact to diminish the potential health risks of working with these kinds of substances.
In the case of selenium, the line between micronutrient benefit and risk for selenosis seems to be at 400 ug per day for adults (see http://dietary-supplements.info.nih....s/selenium.asp -- Table 4, about 2/3 of the way down the page) and about half that for children. That's less than half a mg of elemental Se, which I would estimate to be equivalent to maybe 2 mg of dried KRST.
PE did SeO2 oxidations as a grad student and survived unscathed because he handled it properly and was careful to avoid inhaling or ingesting the dust (i.e. didn't eat in his work area).
My $0.02 only.