How does switching grand companies affect PvPers?

I don't want to test it out at rank 17 and am hoping someone else has since nobody in my guild has either.

A.)If I switch grand company will I start at rank 1 and if I switch back will I then be my old rank?

B.)How often can I switch between the companies?

I'm basically wondering how feasible it would be to say be with the Order of the Twin Adder on my White Mage and Maelstrom on my Bard to keep seperate PvP skills available to each and just swap back and forth depending on what I was in the mood to play.

a.) yes.
b.) every 30 days.

one thing to keep in mind; eventually when frontlines comes out, there will be specific pvp abilities for specific gc's. it isn't a bad idea to get your rank up in multiple gc's in case there are abilities in a certain gc that are more useful/geared toward your play-style in that gc. source: some live letter from a while ago.