If I add a grenade launcher dude to my Fleet Trooper Unit, can he use the standard blaster pistol instead of the grenade launcher?

Yes. All the minis in the unit have the unit's organic weapon, just as they can attack with the unit's organic melee weapon.

You can find this info under "Weapons" in the RRG.

"Personnel and heavy weapon upgrades can add trooper minis to a unit. While attacking, these minis can use the weapons of the unit card they are equipped to."

So for example the new Imperial Comms Technician if added to a Stormtrooper unit, gains all the stats of a stormtrooper?

Thanks in advance

It would be different if it had the upgrade keyword Sidearm: Melee/Range. For then it may only use that weapon in the way listed in the keyword- Melee or Range. An example would be Imperial Royal Guard's electrostaff guard upgrade which has Sidearm: Melee. That one miniature among the unit may only use the electrostaff during Melee combat. It still can use the default ranged weapon on the Imperial Royal Guard card doing a ranged attack.

4 hours ago, zsavk said:

So for example the new Imperial Comms Technician if added to a Stormtrooper unit, gains all the stats of a stormtrooper?

Thanks in advance

Yes, and the officer is the same.

Somewhat disappointing in this regard that a Corp unit Officer upgrade doesn't get to use it's RK-3/A-180 pistols. Especially for a closer range uint like Fleets or Flamer-Snows. Unless it does and I'm mistaken.

It doesn't unfortunately as it is a commander unit card while the other is a upgrade card adding an additional mini (which in this case is the imperial officer). Same mini with two different roles, keywords, and stats.

I agree with you to a extent, it would be nice if it had its weapon yet if was exactly the same it heavily lessens the point of having it as a commander with very little exceptions.