Glycerin suppositories will empty the rectum, but generally not above. 6" should still be comfortably inside the rectum (the transition to the sigmoid is usually at about 7"+)

Before trying anything test to see if you need to do anything. Your digestive system is usually on a schedule, and if you time your play right a number of people find that they don't have any problems. Test things out with a finger or dildo and see - it's best to not mess with things if you don't have to.

If you do want to do something to make sure it's clean then again test out what works best for you. Make sure to use only water or water with a little salt or baking soda if you're taking an enema/anal douche.