Says right there in the codex that you may replace any weapon on a ravager with a Disintegrator cannon.

As for the Draigo+ Paladins idea: REALLY cool thematically, my force would shred it in seconds.

You lose 2.5 paladins per turn. After turn 2, you are realistically looking at only Draigo himself on the field. And the only ranged anti-tank anyone in the army has in that list is the S5 AP- large blast template, which is unlikely to hit anything at only 12" range. Remember, ravagers are AV11 so the storm bolters aren't going to do jack.

Place the ravagers in front of the Venoms to create a LOS-blocking, cover-granting wall and frankly that force is just going to crumble without any ability to actually engage anything in CC and outranged completely.

Even if you deep strike, you have to hope that you roll 6s all over the place (AND that I fail cover and flat-out saves).