Some people are a bit sensitive about their own missions, while others are not. Personally I'm not, I would be flattered if you took any of my missions to bits...... As long as you didn't subsequently pass of any of it as your own work, which is what people are worried about. I'm not suggesting for a second that you would, but apparently it has, sadly, been known to happen.
The Tutorial Mission in the Ed Depot is specifically designed for taking to bits. snYpir wrote a mission called Operation Axe Point on which he based a tute, so that you can also delve into. And of course nobody minds you looking at the official BIS missions, all of which are accessible if you have a good un-.pbo utility.