I don't think the Fretmentor cares whether you use fingers or pick, as long as you get a good "boom/chuck" in the rhythm. Though he is a bluegrass purist,he may prefer the pick.
Actually, your thumb and 2 finger routine is preferred by the fretmentor. He says it's simpler, and you can anchor the un-used fingers to the instrument to keep your hand in position. I don't play any fingerstyle, it's something i need to work on. (when i'm done with our songs of the week of course, and our assigned song for the end of the quarter)