Since I am writing quite a bit of stuff for Concert Band, I just ordered a score to Holst's "Songs of the West," which I heard a performance of on FM Radio. It sounded as if it were something from which I could gleen a lot of info on band scoring. We'll see.
Bartok's Concerto for Orchestra, Music for Strings, Percussion and Celeste, and the String Quartets are a high-valued and much-used trio for me. Also all the Beethoven symphonies for 'workhorse' basic orchestration.
For color, pretty much anything by Ravel, IMHO, (Le Tombeau, Daphnis, Valses Noblis et Sentimentales, etc) and I second the large references to Stravinsky. Most know his ballets, (Firebird, Rite of Spring, Petrouchka)... I just picked up a copy of Le Noces, which I find to be an amazing work...
To contrast Rite of Spring, consider adding Prokofiev's 'answer' to it, his Scythian Suite, (the last movement is particularly striking), and his 5th Symphony....
For aleatoric, what about Cage or Carter? Admittedly not a Cage expert here, but Eliott Carter's Concerto for Orchestra is pretty hot. Don't know if it's quite 'aleatoric', but... (Might be a bit hard to find)
Speaking of hard to find, where might one acquire Smalley's reference scores?
PS: Every composer should know about Dover Publications I work part time in a music bookstore here in Boston which sells stuff directly to the BSO. More often than not, I've seen BSO folks leave the store with Dover stuff in tow. Simply the best bang for the buck in terms of large renditions of the repertoire...
For studying orchestral colour I have The Rite of Spring and The Planets, as well as nearly everything in Ligeti's catalogue. My favourite score at the moment, though, is Jan Sandstrom's 'Motorbike' Concerto for trombone.
For aleatoric scores you should check out Witold Lutosławski's Symphony No. 3 and Jeux vénitiens (Venetian Games). John Corigliano's scores often contain aleatoric parts but may be "more accessible" - check out Symphony No. 1, Three Hallucinations (from the film Altered States), and the The Red Violin -- Chaconne for Violin and Orchestra.