Randy,
10 years ago I was attempting to design a pvc central collection system and gave up! I needed Y's, reducers, T's, elbows, etc, etc! And since I didn't know much about fluid flow I was perplexed on when to reduce the pipe diameter. I gave up!
Called my woodworker cousin for help and he steered me to Oneida in Syracuse, NY. VERY helpful. They designed the pipe collection system with proper sized reducers etc. Gave me a quote on the metal pipe system and a cyclone/cartridge filter system. I bought it, installed it, and glad I did after 10 years. They make their own fittings. For example I have a reducing Y that goes from a 6" dia to 5" & 4". other configurations as well that you will just not find in pvc. Back then there was a charge for designing the collection system but it was refunded if you bought the system from them.
A word of caution. You should pay particular attention to grounding with copper wires. Some will poo-poo this as not necessary but believe me it is. I just toured a wood shop last week. The gentleman makes early American furniture and he has a self designed pvc collection system. He said he had to go back and install grounding because he was getting 6" arcs of static electricity! Don't know exactly how he did it but evidently he has copper wires inside the pvc pipes now.
A further note: static electricity is the #1 safety consideration in petroleum refineries, chemical plants, and grain elevators. More explosions are caused by static electricity than any other factor.
Here is link to Oneida: http://www.oneida-air.com/
Good luck. and be safe.
bb