You won't get those fittings hot enough to affect the solder in them, but if it will give you piece of mind, use the wet rags around them. If somehow a joint does become hot enough to melt the solder, coat it will with flux and reheat it. If there isn't any flex in the line when you add the tee, use the tee as a template to see how much to nip off. To clean a pipe when a fitting is removed, use a rag while the solder is still molten. You may want to do this a couple of times, then emory cloth or sandpaper. The major problem area will be the lower side of the pipe where the melted solder runs off. It's a small problem, but not impossible. As far as how much flux to use, don't be afraid of using too much. I probably use 10xs more than necessary, but it's cheap.