Ive already commented but this is incorrect.
If you google and look into this, you will find that they CAN specialize in being a Heavy Gunner. But if you look, The 91st had the following:
If I take key points from this, First of all, They are standard clone troopers:
There equipment is as follows:
The 91st is a corps that troops specilized in RECON, Heres what the 91st troops went through:
Now you're correct in saying all troopers can go through the futher training to get specialized in being a heavy but this is the following for a Heavy:
They get specilized in for instance heavy on Kamino and then they get sent to a battalion for instance the 501st or something along those lines, they dont just happen to be able to pick up a z-6 when ever they please.
Update as I do not want to reply on here again:
EA has stated since BF1 that their Campaign is Canon, their Multiplayer is not. Developers have even stated that it should be treated as a Sandbox style Multiplayer, the event we make etc.
The heavy is a MULTIPLAYER UNLOCK SAME WITH THE SKIN. Slow walk speed, more damage from things explosive related. Does not sound like the 91st at all.
Its not distasteful, its the truth. We either follow lore or we dont, we cant mix and match.