Massing Cobalts is a bad idea. Massing Javelis LRMs is significantly better, of course a balanced fleet of light carriers, heavy cruisers, LRM, and flak is best. If you're facing massed Disciple vessels, mass LRM. They are the direct counter to light frigates which is the category under which Disciples fall.
Capitals are generally bad targets early on. That said, if you're facing a Vasari Evacuator or a Marza Dreadnought that is getting close to level 6, you'll want to focus on them. Later in the game when capital abilities really start augmenting fleets, focusing fire on a capital is a much better idea than early game when your DPS is significantly lower.