The tubes can be used with any lens that'd fit the body without them, and they can be stacked to give as much magnification as you want. Yes, longer tubes will give more magnification.

Note that there are now (I think) different, much more expensive tubes that allow AF on some of the Mamiya bodies. You don't want those. Not much point in AF when you use extension tubes, IMHO. Just get the plain old extension tubes and enjoy. And I don't think I ever got any of my AF mamiya lenses for my AFD to work on the old tubes; I vaguely recall contemplating a modification to the lenses but then forgot about it and just used the manual lenses.

So in summary: manual lenses + tubes = no problems.