Just quickly, what is the difference between these variants in game? Are the skins fully compatible?

Also, did the Free French AF fly them in Tunisia?

The skins should be compatible, A-20C's & Boston III's had only internal and power plants that were different.
And as for A-20's for the FFAF the answer is yes.
http://www.axis-and-allies-paintworks.com/e107_plugins/forum/forum_viewtopic.php?1737

Regards Duggy

Hey Kiddo, PM me and let me know exactly what you need.
Cheers Chris

Cheers lads. I'm switching everything over to the A20C in all missions to minimize file size. As it was, I had a real patchwork of types, all adding to my confusion.

The in-game A-20C has wing pylons and Boston III does not. Other than that, I think there are some minor loadout differences. Strictly visually speaking, the Boston III is a better match since the Boston/Havocs didn't normally carry wing loads during the Tunisia campaign.

Skins are 100% compatible.

Duggy, the French Armee D'l Air (and the Vichy forces later on) used the DB-7, which looks a lot like the A-20/Boston, but can be easily identified by the smaller vertical fin and rudder. The DB-7 was originally built for a French contract and was later adopted by the RAF and USAAF. The RAF also received some DB-7s built for the French but not delivered due to the armistice. They were called Boston I.

IIRC, the Free French used primarily A-20G/Js.