Plumbing fittings and sizes can be very confusing, especially when mixing plastic and metal pipe, but always remember this:
The threads and sizes of PVC fittings are exactly the same as iron (or galvanized) pipe fittings.
The threads and sizes of CPVC fittings are exactly the same as copper pipe and brass fittings.
Those fittings are made to mate with each other and you shouldn't have to be using any weird Rube Goldberg gadgets to make the connections.
When screwing a plastic fitting to either a brass or iron fitting it's VERY easy to cross-thread it, so you have to pay extra attention when starting the thread not to do that.
If you cross-thread a fitting it won't matter what you put on it, it will leak.
If you don't tighten a fitting enough, it will leak. They need to be a lot tighter than you might think.
You've either cross-threaded the fitting or you simply didn't tighten it enough.
Take it apart and start over.
Threads on the plastic fitting may now be damaged, so you may have to replace it.
And when properly threaded and tightened using teflon tape there's absolutely no reason to gunk it up with anything else.