I tried a Pro-Mag. They aren't pretty but functioned. I wouldn't trust their quality to be anywhere near a factory magazine but on a sample of two, they worked.
Yes they do fit in the buttstock mag holders... however, you won't have any room left for your wrist to hold the pistol grip. Perhaps you could put a single Pro-mag on your non-grip hand side and a factory 23 rounder on the other.
As others noted in the other thread, ETS polymer mags will not lock in the buttstock, nor will they function in an FPC due to the thicker plastic mag lips interfering with the bolt. Apparently the slide in a pistol has more clearance than the FPC.