I'm designing a paid tutorial website for pro video software. Is it legal for me to use the actual application's icon in the design?

I don't see why not. I have in the past. As long as it doesn't appear that you are claiming credit for creating them, I believe it's fine. I mean they are all over your computer anyways.

yes you can use them as there is nothing wrong you are doing. You are selling their products or tutorial. You will not be saying their products to be yours...