does a stiff racquet have to use low tension strings?

Answer 1

Stiffer racquets are harsh on the arm. Lower tensions are less harsh on the arm. There's a balance. Most people will use a less harsh string like a multifilament for strings on a stiff racquet. It's about tension and string properties.

Answer 2

no, a stiff racquet can take any range of tension. Although I would suggest that it can't take anything outside of a range of 50-70lbs.
you should check the information on the inside of the triangle of the frame to see what tension it can take. If it can only take a low tension, then it is not a stiff racquet, but a flexible racquet.
ask for more details if necessary.