Hsing-i is tough on the knees when done right, but that why you can hit with your whole body.
Knees need Glucosamine chondroitin, and silicate.
Silicate is contained in steamed okra and bamboo shoots (raw and cooked). Lecithin will lubricate the joints.
Deep knee stances are good if you can do them, but not required.
Classic San Ti standing of 1/2 -2 hours per day for the first three years is the standard.
There are other San Ti stances that will help, without the knee stress that are detailed with photos in the book described at bottom.
Try doing San Ti, without bending the knees so much. Another method is doing it in water that is waist to solar plexus deep.
Doing Hsing-i Metal and holding axe hand at end for three breaths, is described in Robert Smith's book.
Metal, and Sparrow use San Ti, and Dragon uses a reverse form. One can do these slowly also.
Exercises that will help: knee circles and bending the inside as in lotus, and outside with heels outward and inside of leg on ground.
My newly published book will help with the Hsing-i training and feeling Qi; Axe Hand; Hsing-i & Internal Strength Workout:
available at publishler Lulu.com or at amazon.com