Pretty much agree with Michael Chandra... but some additional thoughts:
1) it's not very fun for the player to force the controller to continually come manually feed a new belt/clip, despite having a 250 "ammo capacity". Far easier and I daresay fairer to just figure the weapon mount includes whatever switchers/feeders to keep the gun going remotely.
2) there's no reason they wouldn't.
3) I'd probably say if the underbarrel weapon is intergral (so Raidens, Alphas, yeah those kinds of things) it's reasonable to say the same Targeting Autosoft governs both. However, it's also reasonable that they don't and you'd need a 2nd Targeting Autosoft for the UBGL. So YMMV... make sure you know what your GM thinks. And as far as I'm concerned, if the UBGL is an optional add-on, then absolutely you need a seperate Targeting Autosoft for it. Either way, the weapon mount only gives ammo capacity for the "primary" weapon: bullets for the rifle. My understanding would be that the UBGL has an ammo capacity of only what it can carry itself; the weapon mount won't add to this. You want to fire 250 grenades? Then install a grenade launcher, not an UBGL under a rifle.
4) As MC said, you used to have to run a special autosoft for drones to make use of smartgun technology. They didn't include that this time, and I choose to NOT believe it's change blindness but a deliberate change for streamlining/ease of play. If it were up to me, just let the smartgun do what it's meant to do without a program tax. Provided you do bother to install a smartlink vision enhancement onto the drone's sensors, of course...