Ron,
I'm Canadian - Canada develops a new National Code every 5-8 years and then each Province adopts this new National Code for their Provincial Code with certain amendments, but the latest edition of the National Code was so good that there are very, very few amendments for each Province I'm told. You will find if you ever have the chance to read the Canadian National Code, that is as good or better than what you go by seeing as how Canada typically deals with some pretty harsh environmental conditions for the most part. Having said that I"m not sure if in our Plumbing Code, such a rule exists, but I do know that Inspectors will fail an inspection if you screw a metal into a plastic if the plastic FIP isn't designed for said purpose.
Ipex has a PLASTIC Female adapter that is prestressed with a metal ring around it. This fitting is designed so that you can screw metal MIP fittings into it. They make them for both Sch 80 PVC (Xirtec) and Sch 80 CPVC (Corzan) as well as for their Aquarise (CPVC) lines. These fittings are the only fittings that will tolerate swings in temperatures and not leak made by Ipex anyways, that I know about. I'm sure you've probably seen other manufacturers that have a brass FIP that is impregnated into the plastic adapter too. The Polypropolene adapters & saddles I fuse (Fusiotherm/Aquatherm) are of the type with a brass FIP impregnated into a plastic adapter.
If you screw a plastic MIP into a Metal FIP, the joint WILL leak if it is expanded and contracted due to temperature change of the water in the pipe. This is mainly in heating systems, but could possibly happen in a hot water line where CPVC was used.
Whether or not Wet N Dry is the right glue or not is not up to me. I was just merely relaying information more so than anything. In a lot of cases, you should be pretty safe following manufacturers instructions/specifications and that is what I tend to do.
Apparently North America in general is known for having fairly tight codes, but looser restrictions on products, where as in Europe that have very tight restrictions on products - they don't get sold, and a bit looser restrictions on codes.