What about this. In the cruiser section: "you may not have more cruisers than escort squadrons." In the light cruiser section: "you may not have more light cruisers than escort squadrons." That way, escorts are the foundation of the fleet.
What about grand cruisers? They are available in the RT RPG books.Grand cruisers are "surprisingly common among the wealthier Rogue Trader fleets," says Battlefleet Koronus, the book. Also, battlecruisers are more rare for RTs to get, but some do. The 2010 RT exploration list gave a way to get a battleship in the list, but it was near impossible to do. To do this, you needed yo max out your cruiser capacity with exactly 9 RT cruisers, 3 cruisers of allies, THEN you could take 1 attending battleship to go along with it. Doesn't seem right to have it so rigid.