Yes it is important to differ between Combi's and QC's
The QC have very little legislative problem as they either comply with one set of rules or the other ( pax/freight)
I have worked with both DC QC and B737 QC and I have to tell you that this scheme is great on paper
A typical operation was night freight - Quick Change - Day time pax.
After some wear and tear the "QC" beocomes a mute point which manifests itself in larger hammers!
Also the interior gets beat to Sh1T and many manhours get spent getting all the pax stuff working.
Additional with weather and loading delays it was always a hassle to get the pax version on line in time.
Re Combis, On cargo aircraft there are certain requirements for the cargo compartments for fire access to the load and air shut off in the event of fire.
These requirements effectively double when you go combi.
You cannot shut all the air off to the cargo compartment as the pax need some of that
If you have extinguishers then you have to stop the pax getting a dose of that
On a wide bodied aircraft the pallets sit in two rows effectively touching in the middle (Well very close
)
On combi there has to be acess for the crew to the load in flight that gives access to all sides so the pallet spacing has to be increased.
Of course as we mentioned above not all registers are so picky !
There is of course the 9G bulhead ( either solid or net) and usually the pax are at the back for both a load consideration and to prevent them wearing the cargo in the event of the crash.
As I said the rules have tightened considerably - When I was working in Africa on DC4 we just has a few rows of seats behind the cargo ! and the pax numbers could be adjusted to suit the cargo size !