You can have less than a K’beitza of bread/Mezonos and as much of anything Shehakol or Aadamah as you want outside the Sukkah. It’s best to eat absolutely everything in the Sukkah though (even water) if possible. Using a neighbors Sukkah in theory shouldn’t make a difference but probably does in regards to the Chumra of having everything in the Sukkah.
You can eat anything inside if it’s raining/too uncomfortable to be in the Sukkah for a variety of reasons.