1. I currently have one big, red gas line coming in from the tank. This line measures 5/8" OD and connects to the sankey tap. While shopping for a 2-way manifold, I find that they come in 5/16" & 1/4" sizes. Which size do I want? Neither seems like it would be able to fit my line.
Not sure but I'd guess the 1/4" would be closer fit, but either will be tight...tight is good. You're likely to need to dip the end of the hose in boiling water to soften it so you can slip it over the barb.
2. This same question applies to ball-lock disconnects. (I've decided to go with ball-lock kegs)
Go with 1/4"
3. what are your opinions on barb vs MFL fittings.
Absolutely go with MFL. Life is so much easier when you have to remove the disconnect to clean. Otherwise, each time you want to remove the barb connection, you'll end up having to probably cut the hose because it's so difficult to pull those connections apart.
If it makes any difference (or if it's even possible), I would like to be able to serve both sanke & corney kegs with a minimal amount of cutting/refitting between kegs.
Go with one system. It'll simplify your life a lot.
Any suggestions would be much appreciated!