Beautiful Classical Smaller Pieces, duo,trio,etc

Many great suggestions. Glad to see mention of the Nielsen Quintet and of
Janacek, one of my favorite composers. Janacek's "Mladi"
(Youth) for wind sextet is a gem and highly recommended. Schubert's
"The Shepherd On The Rock" for soprano, clarinet and piano is
one of my favorite chamber works; as is his "Octet" for winds
and strings. Poulenc's "Sextet" for piano and winds is fantastic.
Prokofiev's "Sonata" for flute and piano. In a more modern
vein (beauty is in the eye/ear of the beholder):

Ligeti, "Ten Pieces" for wind quintet.
Elliot Carter, "Sonata" for flute, oboe, cello and harpsichord.
John Adams, "Road Movies" for violin and piano.

Schubert, I have been privileged to play several of Janacek's works over the years; most recently "Sinfonietta". "Mladi" was a staple of my woodwind quintet's repertory; with the addition of the bass clarinet, of course. Janacek was one of the most individualistic composers in all of music and while his music is, as you point out, dissonant it always remained tonal and rooted in Romanticism and the folk music of his native Chekoslovakia and of Russia. For me, one of the most fascinating aspects of his music is that he is sometimes referred to as the first Minimalist composer with the use of short repetitive or recurring motifs throughout a composition. Truly a musical giant.