[@Necroes] plus the crew of the sloop is about 7500.
And it's less than a kilometer long.

As for ships being able to hold tanks.
Sure, if they are however, nah.
The warships are usually mostly warships, the imperium often have troop transports for ferrying guards.
Space marine vessels are different, but they are space marines.
In BFG, mechanicus even had a special rule they had very little troops on board, so they got a penalty on boarding actions.
So tanks are usually not on navy vessels. Since they are completely different military arms of the imperium.

As for guardsmen getting the information they need.
Have you seen the imperial infantryman's uplifting primer? How to fight an ork mega armoured nob?
"Just shoot it in the head and it will drop dead in an instant."
Genestealers.
"Genestealers have puny claws and move slowly."

It is that kind of downright lies the imperium tell about its enemies, when they even tell of them at all.
Guardsmen who know of chaos space marines have learned about them from facing them.
Sure, Vespin might have faced CSM's and lived. Being a sniper helps there.

And the imperium doesn't always mind wipe people who know of chaos.
But chaos and the warp is actually far from common knowledge in the imperium. If someone mentioned Khorne to someone, they'd probably have no idea who or what you were talking off.