Do not connect a secondary regulator directly to your CO2 tank! Secondary regulators aren't able to handle tank pressure (~1000 psi), so you need the first one in line to be a primary regulator to step the pressure down to something they can handle (they're called SECONDARY regulators for a reason). Some sets of regulator bodies sold as "secondaries" are actually just primary regulators daisy chained together, and actually can handle tank pressure, but that's a dangerous assumption to make. If you only want 3 pressures, you only need three regulators including the primary. A primary and a 3 body secondary set will get you four pressures.
The only "right" volume of carbonation for a particular beer is the one you like it at. The suggested carb levels in the style guidelines are just that, a suggestion. If you notice, most style guidelines give a fairly wide range of carb levels for each style, and most of them overlap somewhere in the middle. You'll also get significantly different carb level suggestions for the same style depending on which set of style guidelines you look at.
I'm happy keeping the vast majority of my beers at the same moderate carbonation level (~2.3 vol for me), so I only use one regulator and keep all 4 of my kegs at the same pressure.
If you really want to fine tune the carb level for each beer, then multiple regulators are the way to go. Just make sure you cut all of your beer lines long enough to balance the highest carb level you foresee yourself using (or even longer IMO). If you balance one of the lines for a mild at 2.0 vol but then try to serve a hefe through it that's carbed to 4.0 vol, you'll pour nothing but foam. If all of the lines are on the long side, you'll be able to get a good pour with any beer, it will just be a little slower for the less carbed ones.
