Intercessors use old legion rules and formation tactics. Which means the entire squad takes something or no one does. 

I.e we all use bolt rifles, or we all use stalkers, or auto-bolt rifles. 

As for the grenade launcher I kinda already claimed that with JB, as there is typically only 1 launcher per 5 marines. its the only auxiliary equipment intercessors use. 

Unlike standard tactical marines which are purpose built to be as flexible as possible with options for wargear. the Primaris are more purpose built. 

But ya, the sergeant cant take a power sword. I just assumed JB would be taking on the role of sergeant