: Is it on sale? This sentence is fine.
:Do you guys have soy milk? Yes, we do. This is also fine, very natural.
:I usually start working at(from?) 3:00pm. "I usually start work at 3pm" or "I usually work from 3pm until 10pm." Only use "from" if you will say when the end will be.
:Can I switch rice to noodle? (When we order food at a restaurant) This would be fine to say but it would be better to say "Could I have noodles instead of rice?"
:Who inspired you to study Japanese? This sentence is fine. ^_^