The cap on the coolant filler neck is looking worn. I have a nearly new pressure cap and wonder if I could use that or would that cause problems.

I have not even checked to see if it fits.

If not, anyone know the size of the non-pressure cap?

 

I wouldn't use the pressure cap, but if it fits it shouldn't do anything because there is no ridge for the plunger to seat on in the filler neck. Dang how much could a replacement from the devil err.... Mazda cost

 

They both have the same gaskets at the top seal so not really but the cap for the filler is only about 15$ ???

 

There is a ridge on the engine filler neck, and you could technically use the pressure cap as long as it holds a greater pressure than the AST cap (Though I don't see why it alma matter since there is no where for the pressure to go even if it did overcome the spring tension.)
I have run my car with a pressure cap on the filler neck without any ill effect, but I purchased a stock cap just for good measure at the price mentioned earlier.