It's fairly normal for the action to change, a little, when you change string gauges. Normally, it gets higher with heavier strings and lower with lighter ones. A silk-and-steel set with an 11 high E, and the others in proportion, is about as little string tension as you're likely to find, and it's not at all surprising it would create a low enough action to buzz. Best thing to do is decide what you like, then have it set up for them specificly. Or to I mean "specifically?" I can never remember.

Keep in mind that humidity changes have a big effect, too. I go crazy trying to keep my guitars' actions right, in a house in Vermont with partial wood heat, that gets damp in Summer and dry in Winter.

Peter.