I can put normal modules in weapon slots, and otherwise. All in all - any module fits any slot.
Mission 1. Federation. Frigate hull “Puma”.
Not tested other races or hulls or hull sizes.
PS: shield generator 2 fits weapon slot.
Half of that behaviour is correct: Ship modules are supposed to fit in hardpoints (weapon slots).
I cant reproduce incorrect behaviour. I can’t put weapons in no weapons slots. Can anyone?
can’t reproduce here 
Not here either.
Then delete/lock this topic as it seems it is invalid. Maybe I had hallucinations 
I wonder if he means putting non-weapon items in weapon hardpoints.