As a twenty + years Corrections professional, in my humble opinion, the less skin my staff exposes, the safer they are. I don't want to see staff exposed or being exposed to more things than you can think of while on the floor wrestling with inmates, be they kilted, females in skirts, or staff wearing shorts. Long pants are not the most comfortable, but barring a full body suit, they are the safest.
As for visitors and professonals (attorneys, clergy, etc) they can wear shorts, so I can only assume that kilts are allowed. So far, no one has tried it. My husband has come in a few times kilted to see me, and the inmates just stared. The staff thinks he is cool.