Important question, to me at least!

Ok, so, since I am starting up a band, I figure it's a good time to get back into my theory. I know I can play a minor/major pentatonic in any major/minor key, that's nice and all but I would like to know more! Like as to why? My question I guess is really based more around the relative minor of a major key, and does it work backwards?

Also, dealing with major scales, can I play them in the key of the relative minor? What about minor scales in the relative major??

Hopefully you guys know what I mean, you're all pretty smart so I think you do! Thanks in advance if I don't get back after awhile.

:D