First question- have you read
Bill Pentz site?
There is a lot of good info out there, and I am a long way from being an expert but have done a lot of research and put DC's in two shops. Here is info minus the technical jargon-
You need to decide if you are using a DC to collect chips and dust, or using it to collect chips and dust AND have healthier air in the shop. The last point is the tough one.
If you live in an area where you can discharge outside that is best for both issues. If you are using a DC with a bag (bad with regard to point 2) put the DC and bag outside. That helps with the noise too. What kind of separator do you have? A "true" cyclone is much more efficient at separating and better on flow than a trashcan separator. If you can, get a cyclone with cartridge filter(s).
As far as running pipe.
Use PVC (S&D, ASTM 2729-
NOT BORG schedule 40!!!). The price, especially fittings, is cheaper than metal.
DON'T glue it, you
WILL reconfigure someday!!! TRUST ME ON THIS ONE!
Run 6" as far as you can, reduce only at the machine. Enlarge the machine outlets if you can and don't mind modifying your machine.
Use as little flex hose as possible.
Make all runs as straight as possible.
Limit turns since they impact efficiency.
NO T's. Use wyes and 45's. Use 2 45's or long sweep 90's if you must.
Don't worry about multiple branches, what counts is the how straight the path is from each machine to the blower. With a 2 hp DC you'll only be using one machine at a time.
If your tools are along the wall, running the pipe at 36" off the floor should be fine and actually better than overhead- no vertical runs. Remember, wyes, not T's.
Build, don't buy blast gates- you can easily build them better and cheaper than commercial ones.