I'm not entirely certain, but I believe the Belgians used the original pattern of Adrian helmet (M15), while French had a new version (M26), which had a narrower brim, the difference being most noticeable at the front. There were other minor differences, like ammo pouches and the Belgians wore leather gaiters and not cloth puttees.
I doubt much of that is really noticeable in 15mm. Cut off the top mag on the LMG and I'm sure it will pass for a BAR. You might even have look for some Great War French and see how they look.
Personally I'd just use them straight out of the packet as is, the Adrian is the key identifying feature and as Fred says, the WWI British look of the officers; although they too would usually wear helmets in action.