Go to your container's WebSite, look up your container and look at the specs. If you container manufacturer's specs say it's O.K. it doesn't really matter about anything else, now does it? You wonder what has happened to "peeple" who have done it, i doubt you'll find that caliber of people on this web site. They're all well above the average intelligence level.
Assuming a 'normal' ram air reserve system, if the reserve doesn't fill the free bag the locking stows may not/won't have any tension on the lines. This could lead to an appropriate deployment sequece. In addition the container could not be full enough to properly contain the bag, pc and bridle and in the instance of an external PC it may not protect the PC (seat the cap) well enough to be safe.
I had a rig brought to me with a Tempo 170 in a J5. Another rigger had assembled and packed it. I wouldn't pack it. There was absolutly NO tension on the locking stows. A third rigger did pack it, saying he'd do it once but it should be changed. I know he did it at least one more time. Then the owner became a rigger and kept packing it. THEN he sold it to a newer jumper, who brought it to me again. And again I had to explain I didn't believe it was compatable and wouldn't pack it.
There is range in container sizes and reserve volumes. Not every combination that is supposed to fit will. Some will accept a smaller reserve more easily.
Adding 'stuffing' to a reserve container won't solve the problem with the locking stow or loose bag around the reserve pack. I have yet to see or hear of someone trying to stuff the free bag itself. I suppose you could try to put a smaller freebag in a rig but have fun trying to convince a jury it was compatible.