If you are going for 6 levels in any of either class, I agree on chosing the monk - you just lose too much by going Rogue 6 in my opinion.
Rogue 5, however, offers Vault, which is quite useful, and 2 sneak attack dice compared to Monk 6/Rogue 2. Still, you should be fine either way.
As for epic destinies, don't stress yourself over those, yet. The build should work well with Legendary Dreadnought, but that can wait 'till you hit 20.