[martin417]

There are very few restrictions on the use of "fittings", just be careful in defining a fitting. For example;

A quick exhaust valve is not a fitting, Nor is a check valve, but a 1/4" NPT close nipple is.

Also keep in mind that the 1/8" NPT restriction is for valves, not fittings. It is allowable to use fittings larger than 1/8" NPT, just not valves that have ports larger than 1/8" NPT.