You should buy your manifold to accommodate the number of kegs that you plan on using. If only two kegs, then a two-stage manifold, etc.
The diameter of the MFL fitting for the manifold is of little consequence. You could use 3/16, 1/4, or 5/16. Any will work well for the CO2. However, when you purchase your CO2 lines, you will need the appropriate tail piece to fit your CO2 line diameter. Again, your CO2 lines can be 3/16, 1/4, or 5/16. Some people on this forum use 3/16 diameter for all of their lines so that they can have tubing on hand to work for either the beer or the gas line.
As far as having your beers at different pressures, there are a couple of options. If you want your beers at only two different pressures (I am assuming that you will have only two kegs), then you can go with a dual body primary regulator and skip the manifold OR you can go with your Primary Regulator that you already have and use a dual body Secondary Regulator.
If you want more then two beers, but again only two pressures, you can go with a Dual Body Primary Regulator and two multiple manifolds OR you can keep your current Primary Regulator and use a secondary regulator that will accommodate as many kegs as you can fit in your freezer.
I use a single body Primary Regulator and a four-body secondary regulator. What you do will depend on how many beers you want in the keggerator and how much money you are willing so spend.
I hope that this helps.
Mark
