The Istria Class Corvette is normally used to mop up fighters or patrol rebellious systems. It's armed with two Derangium shells in case it needs to orbitally bombard something or fight a larger vessel.
Partizipazio Class Heavy Cruiser
The Partizipazio is the most effective weapon there is against pirates. Fast, heavily armed and with decent armour makes it ideal for finding and shooting the shit out of Venice's enemies.
Achaea Class Floating Fortress
Primarily used as defense satellites, Achaea's were designed so that it had no weaknesses in armour or firepower. The only problem is the lack of mobility.