Warlord is right about the smoke, the highest penalty applies, not the cumulative total of all penalties. As far as MP is concerned, my gut reaction is to say no they do not stack, but I don't have a hard and fast rule to quote off the top of my head. However, since MP does not stack with cover, I don't believe it will stack with smoke either since if it did it would be more difficult to hit a target behind level 3 smoke in MP than it would be to hit a target in hard cover, which doesn't seem right to me. Anyone have an actual rule or page number to quote this gentleman?