Dumb question inbound, please understand.

I've only played like 3 games, so I'm confused. I was told during the Shooting Phase, if models in a unit have Pistol, they cannot shoot if everyone else does and vice versa

Example:
>6 Noise Marines per unit, Disharmonist has Pistol
>Kakophonist has pistol

>the Disharmonist and Kakophonist shoot, the 5 basic Noise Marines cannot shoot as the Pistol Keyword is not present on their weapons.
or
>Sonic Blasters/Blastmasters shoot, the Screamer Pistols cannot as they have Pistol and the others don't

I thought you'd only pick if the model had a pistol and a regular shooting weapon?