You are right that the Parker style ballpoint and roller-ball refills are interchangeable but I haven't heard of any roller-balls that would work for a cross style ballpoint.
I guess the words Rollerball and Gel ink refills are not interchangeable. I was referring to the Ops post and said roller ball when I meant to say gel ink refill. I replace a lot of the parker type pens with gel refills which are not capped and they work fine and haven't been drying out per no complaints from customers and from my own use.
Tim.