You need only 2 pressures - One for the beer, and one for the pop.
Beer should be carbed and served at the same pressure - about 10-14 PSI depending on temperature and desired CO2 levels.
You beer will carbonate at serving pressure! It'll take about 2 weeks. You can do it faster with higher pressure.
The Root Beer should be higher pressure (IIRC - I don't serve root beer, so I've never really looked that close). If you want, you can use that pressure for force carbing to speed the process.
The thing to remember is that the carbonation level of your beer is a function of pressure and temperature. If your beer is perfectly carbed at 12 PSI and 40 degrees, and you drop it to 3-4 PSI to serve, it will lose it's carbonation. This works for some people, but I think it's a pain in the ass.
Search the forums for "Balancing" your system - This explains how beer hose length impacts the foam in the glass. This is the correct answer to foamy beer, rather than dropping the pressure.
Search the soda forums for proper root beer pressure. I'm guessing it's 20+ PSI, and therefore quite usable for "Fast" force carbing in less than 2 weeks.
And for a load of ways to force carb your beer really really fast, search for "Force carbing" or something like that. You'll have lots of ideas, and just as many people that will thoroughly explain why it's a bad idea (I do it, when I need to - I'm not hatin