I'd say you have to take TWELVE cruisers, because you need to take 3 cruisers/heavy cruisers in order to take a battleship. So, to take 3 regular + 1 reserve Battelship, you need 12 cruisers.
Because the new Space Marine rules say ANY battleship, grand cruiser, battlecruiser, or heavy cruiser can be a Venerable barge, I'd argue that yes, you CAN use the named battleships as Venerable barges. However, the Powers of Chaos that are outlined in Powers of Chaos would not apply, as it says so in Powers of Chaos (no Chaos marks for you, Space Marines!) As such, you're paying for rules you don't get.