Mixed fleets get focused down more effectively. Say I have 8 corvettes, 4 destroyers, 2 cruisers and a battleship. The enemy fleet will focus their fire on one of these 4 groups. That means they're doing all their damage to say, the cruisers, which are promptly annihilated. Then perhaps my corvettes get focused, followed by the destroyers, then the battleships. Your fleet will do much better if they're all the same ship class as then they'll spread the damage around better.
Hmmm. That defies a core concept of military engagement: fire at everything to keep their heads down and then focus your excess firepower on one or two units to neutralize them. By keeping everything under threat they are required to be constantly in evasive action to minimize the effect of incoming fire, which in turn reduces the effectiveness of their own outward bound fire. Look at an analysis of Jutland, Savo Island or Leyte Gulf.
Small ships tend to hit way above their weight if they can get into range. Large ships can take the beating for the most part. Therefore large ships want to keep small ships at a distance while using their large weapons on other large ships.
I have tried using the battle AI to replace this by using multiple fleets all with combat computers with specific roles. Escort Corvettes and destroyers which stand off and intercept attackers on their way to my bombardment cruisers and battleships. I am still not sure it works though... but it might.
Obviously space based battles are somewhat different and the game mechanics as well, but nothing that cannot be adjusted for. Anyone have any ideas on how I could test this?