When writing for pit orchestras, I always feel that I can get the most done with this instrumentation:
Flute I (Piccolo), Flute II (Tenor Sax), Oboe (sometimes doubling English Horn) Clarinet I, Clarinet II (Alto Sax), Bass Clarinet (Bari Sax)
Trumpets I, II; Horns I, II, Trombone
Violin I (with divisi), Violin II, Viola, Cello, Bass (sometimes doubling Electric)
Harp, Piano (Celesta), Drums/Percussion
However, with other writings, I like to stick with small chamber ensembles.