I have gotten all manners of horrendous matted messes. I've worked with rescues, shelters, abuse cases, body parts being melded to others (ears and tail once), you name it!
I have sometimes needed a #10 to break up the matting in certain parts, (armpits, groin, face) but I have NEVER needed to go shorter than a #7 as an all over blade. Invest in quality equipment and wet shave. It is sooo much safer. If that was me I would have given up and recommended a vet groomer. I won't go shorter than a #10.
Whenever I hear people say, "9 out of 10 forest fires are caused by humans", all I hear is, "Somewhere out there, there is a bear that knows how to use matches."