Hello Gabriel,
1) Vacation is a great time for (or to) people have some rest from their daily routine; You're right: for.
2) If I win a good amount of money, I'll buy a lot of cheap electronic devices or cheap electronics. (?); Electronic is an adjective, so with a noun it's okay.
3) The old lady called me for (or to ) an interview. (?); You're right: for.
4) Music relax (or relaxes) the mind and shake (or shakes) the body. (?) Music should take the third-person singular. relaxes and shakes.
5) Is there any mistake in the following ?
- Music helps me to relax in a difficult moment of my life.; Not wrong, but if you use simple present, then there is more than one moment, so I would say "in difficult moments in my life."
6) Should we say I hate hard (or heavy music)? hard rock or heavy metal music