Well, sure, different regiments would be more and less familiar with different enemy factions, based off who they usually fight. Armagedon regiments would be more familiar with Ork, Cadians with Chaos, and Catachan with all manner of random, xeno species.
However, they're all still educated on the vast enemies of the Imperium. You don't hide that kind of information from your troops, especially when so many of your enemies can just appear at random. Sure, a lot of guard probably haven't Seen Eldar, Necrons, Orks, Daemons, or even Traitor legion... but they'll have been made aware of their existence, because each and every one of those factions could appear out of nowhere, at any time, and the Guard need to know how to engage them if and when they do.@Klomster
I'm not as familiar with the various kinds of ships in the Imperial Navy. I was just speaking from a logistics stand point. It would stand to reason that a ship that can hold over 10,000 people, while still functioning as a ship, could hold a few tanks. However, as has been pointed out, we have no idea what's happening with the ship, where it's going, why guard are on it, etc.
Though, it has been stated, the ship wasn't actually meant to be in this space combat. It was doing something else, and got called in to give aid... somehow, for some reason...