Re: violas, I'll go more into the story I briefly mentioned.
Back in the days when WSS was on Broadway, certain musicians came with the theater -- if you were going to rent a theater for a show, those musicians were part of your pit.
There were 2 viola players in the Winter Garden(?) that Bernstein didn't think were very good. He was told he either has to use them or pay them anyway and not use them. He was leaning towards the latter, but finally decided to write bogus, very easy and slightly unnecessary viola parts.
My guess is when the show left the theater, he just did away with them, and that's why there are no viola parts in WSS.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli