Question 2: Can I use soundtrack from other Bethesda games, like the ones from Skyrim?

Thanks!

You can't use a remix unless you have obtained a license to remix the original work. Yes, that's a thing; songwriters need to be paid too.
Bethesda does not allow content to be used across their games, so no to your second question as well.

Fair Use

Music, lyrics, and music video:

Up to 10 percent of the work but no more than 30 seconds of the music or lyrics from an individual musical work.

www. umuc . edu/library/libhow/copyright.cfm

Got it, thanks! That's too bad, I really want to use them music... Dx