I'm playing a warlock/sorcerer. Can I cast a 1st level warlock spell with a 1st level slot instead of a 2nd level slot using my sorcerer spellcasting instead of pact magic?
Short answer: yes.
From the Player's Handbook on multiclassing, under advancement options:
If you have both the spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the spellcasting class feature, and you can use the spell slots you gain from the spellcasting class feature to cast warlock spells you know.