I've used the Parker Space Pen refill to replace a roller ball on a couple occasions and have found the Space Pen to be as good, if not superior, to the roller ball performance.
As for the size and shape difference. If its a closed end, the length is pretty easy to manage. Otherwise, the adapter for the Fisher Parker style can be adjusted and the length of the over all refill as a result. The thing is that you would need to go over that with a customer so they can do it themselves later. Also, a plug is a pretty easy thing to insert if you don't want to mess with the refill.
Finally, I've found that placing the spring on the front of the refill works best. Sometimes you may have to mess with the spring to make it work right - but its not been much of a problem the few times I have had to do it.
Hope that helps!
Brian