Destroyers would be a combat orientated ship. It would basically be slightly larger than Cobalt frigates, but 2-3X more powerful (firepower & armor) and 2-3X more expensive.
Cobalt Light Frigate:MaxHullPoints 600.000000
MaxShieldPoints 350.000000
HullPointRestoreRate 1.000000
ShieldPointRestoreRate 1.000000
BaseArmorPoints 2.000000
9.5 dps
Kodiac Heavy Cruiser:MaxHullPoints 1050.000000
MaxShieldPoints 600.000000
HullPointRestoreRate 1.500000
ShieldPointRestoreRate 2.000000
BaseArmorPoints 5.000000
18 dps
You already have a ship that's almost twice as powerful as a Cobalt.

Battlecruisers similar in size and cost to regular cruisers, but with more armor/hullpoints/firepower. However, they should be pure combat vessels and have NO special abilities.
"with more armor/hullpoints/firepower" than which cruiser? It should cost as much as which cruiser? Cruisers have pretty diverse cost and function, so a general statement like this doesn't tell much.

1.Make destroyers something weaker then they (crusiers) are now and add crusiers with similar power to those now existing in game.
Why make them weaker? Just reclassify them (if you don't like them called cruisers), keep the Kodiac as a Cruiser and add a second type of direct fire fighting ship. How about an Ion Cruiser that does only shield damage?
2. Make capital ships more pawerful (current balance will be lost :/ ) and add crusiers with this same power as those now existing in game.
Capital ships are already very powerful. I don't think they need much more power.