I don't have my IWB for the PCR yet (should arrive any week though), so I can't comment on that. I did stuff the PCR in the holster my RAMI lives in and I found it comfortable. But that was with it riding a bit higher (trigger not fully protected). Since I favor a cant, I don't think the length impact will be that great. Weight impact is negligible. The most important part for carry comfort is a good holster, not the gun.
Regarding the rattling, as I said in a different thread, I find it to be a non-issue. When I load my RAMI up, I insert the mag, chamber a round, decock, eject the mag, top it off with one more round, and give it a slap. At the time, there is no rattle. I insert the mag, and it starts to rattle a bit. (I think the mag release is nudging a round. Then I holster it, move around, and the rattle is gone. Alternatively, I can shake the gun a bit, and the rattle is gone. That is with a 10 round magazine and Hornady Critical Defense ammo. But I'm reasonable sure the same applies to other ammo.
I think folks just get to worried about "the RAMI rattle". It's not clinking around all the time you walk about. In fact, any holster noise (being is Kydex or leather squeaks) is more pronounced than any sort of rounds rattling in the magazine.
That's at least my experience.