The shorter the barrel the more violent operation will be.
I'd think that under 10 inches and you'd get into damaging the gun and ripping case rims or heads off.
The reason the AKSU has the expansion chamber and trumpet shaped flash hider is to prevent too violent operation.
You'll probably need something similar, but even so, under 10 inches and things will get rough on the rifle.
This will be especially true with a 7.65 rifle.
The AK was designed as a rifle. Converting to any kind of pistol is pushing it. Going shorter will likely ruin the gun.