Pam Tanowitz's new Informal (Guggenheim, March) was a thoroughly uptown experience though Tanowitz is a downtown artist.  In the gilded auditorium, the dancers gave a performance that conveyed classic elegance and taste; one of the musicians even played a gold violin.  Informal, made for New York City Ballet soloist Tom Gold, illustrates his versatile skills.  Tanowitz's eclectic choreography, while full of classical references, demands a casual style and employs the dancers' idiosyncrasies--a challenge for the ballet-trained.  Joining Gold, Tanowitz's own dancers look just as strong as the star, and possibly more convincing, as they glide through here quirky, spluttering phrases.