Update:
I've tried to make it compatible by merging the packs together and taking out the startpos that POR update provides (I'm not sure whether the startpos is necessary to have the population mod).
This has resulted in the game picking up population, however, only certain provinces recognise it - I was suprised to see that lower Italy was recognised, considering there is an extra region...
I can also confirm that manpower works with units.
Unfortunately, northern Italy and Sicily does not show the correct tooltips for individual province growth... only regional...?
Edit: what I mean by this is that the first tooltip for region growth works, but the individual only shows the vanilla 'growth' - this however does work for southern Italy.