I've no idea if they'd let you keep an ACOG at selection. But to me it would make sense to wait until you're at your regiment to have it shipped to you, if possible-- because you won't know, until you get to your regiment, what your assigned weapon will be.You could be issued a HK-416, in which case you're probably golden. But I'm not sure how practical mounting an ACOG on a FAMAS really is, and then there's the FN Minimi (essentially a SAW), the FN MAG (M240); even if the French have rails on the feed tray covers, mounting a standard ACOG on either is a bit silly. Not sure if they're still running AA-52's either, but forget mounting an ACOG on that thing.
That said, I'm a huge fan of the ACOG. I never qual'd higher than Sharpshooter with iron sights or a CCO, but never anything but Expert with an ACOG. The thing can turn a pretty lousy shot like me into, well, something other than a noisemaker.*
*I'm not like, Iraqi bad with a rifle, but I ain't Annie Oakley, either.