Personally, I would follow the manufacturers recommendation but a lot that I've seen put the metal roof right down on the shingles or maybe over an underlayment if the shingle roof is very old(to prevent leaks). Putting it on any kink of strips would make it very difficult to walk on-- if it wasn't difficult already!
Welcome R Jarrett:
I would remove the old shingles for the weight diffrential and start back with the pink building paper. Shingles and regular building felt have asphalt in them and will react to the tin causing a rust through and a leak.
Sometimes it is good to check the deck underneath and do some renailing anyway.