I was wondering is the form of the string quartet's first movement usually sonata-allegro form?
Yes, and the same applies to all movements. See
http://en.wikipedia.org/wiki/Sonata_form#Sonata_form_and_other_musical_forms
Yes, and the same applies to all movements. See
http://en.wikipedia.org/wiki/Sonata_form#Sonata_form_and_other_musical_forms