No they are random events made up of random factions and ships. you might attack and get a fleet of Hapan ships or get attacked and a fleet of chiss ships show up. Haven't seen any Mon Cal raid fleets though mostly Imperial, Chiss, Hapan, Former CIS, or other random assortments. The make up of the fleet is also random. A raid might have three corvettes or several cruisers. They also don't happen every time you attack or defend

If you want to "use" raid fleets while attacking, make sure a pathfinder leads the fleet (raider or CR90, something small & fast). Lead the pathfinder to a corner of the map and wait for the raid fleet to show up. The faction fleet will get distracted and attack the raid fleet. Send your guys in while the faction is still distracted or until the raiders are killed. Defending territory is self-explanatory...part of the faction fleet will likely fight the raiders, giving you some time to take out ships still coming for you. Also make sure to have an HV/ion cannon while defending. I use these methods to give myself that extra little edge while in a battle