First off, I'm dying to see pictures of the beast
Second, if you have both 9x19 and 9x21 barrels, I would stick to 9x19 since the mags are limiting your OAL, not the brass.
I've never loaded for a Beretta, but I do have small frame CZ/EAA Open guns which don't have issues in the mags until 1.18" with RN bullets (FP might be a little shorter).
What weight bullets will you be shooting? I shoot mostly 115s.