Yep , you will need the advice of guys who are casting lead and working with molds. thats not me, not yet.
from my experience, and i could be wrong but i think you will need to use a 9mm bullet that works in the 38 spl.
a .38/.357 bullet may be too long for the 9mm case. but if you get a 9mm sized bullet, you may find one that shoots fair from a .38 spl sized barrel.
i had a few 9mm's that i knocked apart for the primer and bullet. it was a 147 grain, jacketed flat nose. i reloaded them in a 38 spl case with a small amount of unique powder and shot them out of my 357 mag. they worked ok for a plinker load, just to be shooting and fiddling around.
going the other way, using a cast .358 bullet may be to long , depending on what design the bullet is, not giving you very much case capacity left to work with powder wise..but if you are limiting the weight to 125 grains, you should have enough volume to run a descent amount of powder in the 9mm case.