For what it's worth:
I have a original Interams Sig 220 (which is same as yours with heel clip release) and a newer Sig 220 (Exeter made side release frame. The "Sig" mags work in both, however the older heel release hold the magazines about 1/8th inch lower than the newer style.
This makes the mag also sit lower in the frame thusly, with less of the bullet available for the slide to contact as it rides forwards to load a round into the chamber. Although it rides lower, I have never had an issue with feeding any kind of bullet in either pistol.
I will also add that I have changed out the locking insert and barrel in the older model for the newer profiled ramps, although I never had an issue with the older profile.
One benefit of the heel release is that I "never" have issues with seating mags. It either is in or out. With the thumb release, I have inadvertantly seated the mags on the first set of holes in the mags. Why Sig has two sets of mag catch holes on these magazine is beyond me. I assume the mags work in some other pistol, but I have no idea which.
No finally onto your Promag question. On a promag that a friend had, it "looked" like the entire baseplate was plastic, but I did not look closely. It functioned fine in his P220 with side release, but that doesn't mean it will work in a heel release like yours. I would recommend sticking with Sig magazines as magazine are not where to go cheap in a pistol.