You answered your own question at the end where it say “In the finale of his Nintn Symphony, Beethoven took the unprecedented step of using a chorus and four solo vocalists.
The viola is a string instrument that is bowed or played with varying techniques. It is slightly larger than a violin and has a lower and deeper sound. Since the 18th century, it has been the middle or alto voice of the violin family, between the violin and the cello. While the viola da gamba ( can be called a viol/gamba )is any one of a family of bowed, fretted and stringed instruments with hollow wooden bodies and pegboxes where the tension on the strings can be increased or decreased to adjust the pitch of each of the strings. Basically no, but they really do look similar to a violin.
