Are you using 1/2" NPT fittings?
I don't think the polypropylene is interchangeable with the stainless at the 1/2" size. I believe the 1/2" sized poly fittings actually have mating surfaces equal to 3/4" sized fittings.
As you can see here, there are no poly caps or plugs listed in the 1/2" size.
I called them with this issue a few months ago because I had hoped to use 1/2" stainless fittings with poly dust caps and plugs to save $$.
BTW, stainless and aluminum parts ARE compatible at the 1/2" size and the aluminum is fairly inexpensive.