Spoiler: show
It is very uncomfortable moment, becouse at start MP-40-upgrade is hidden by inactive flame-bundle&falame-bomb -> you must to spend 2 CP for "flame-upgrade" -> flame-bundle and flame-bomb will be available -> "hidden" MP-40 will be available.
It is importand, that Luft and TH SturmPioneers can be equiped MP-40 at start!
SUGGESTION
The Flame-bomb is very narrow use (and this upgrade replaces common HE-bomb), becouse common the HE-bomb is good too.
Accordingly, I suggest to remove Flame-bomb from SturmPioneers (HE-bomb will be save) and to add it to call-in Sabotage-squad (5CP!) at start (i.e. without dependence of 2CP "Flame-munition"-ability).
So MP-40-upgrade of SE SturmPioneers will be correct available (Flame-bundle-upgrade will be save too).
Spoiler: show
Sabotage-squad has empty places in screen of abilities for add flame-bomb:
Spoiler: show
I sure, that this replacement will not make great power of Sabotage-squad, but make this even more specialized.
Cost of Flame-bomb 50 ammo for Sabotage-squad instead 60 ammo for current SE SturmPioneers, becouse Sabotage-squad is special behind front-line unit.