Corsairs were space pirates that opted for long range firepower instead of the traditional sword. It was entirely possible these goons were too sloshed on Grog to fire quickly, as they had a slow rate of fire. Corsairs were very similar to Buccaneers in appearance and actions; the only difference was their weaponry.
To kill them, you had to avoid their projectiles, then return fire with a strong long range weapon. They were members of Captain Slag's crew. Rusty Pete referred to himself as a corsair at one point, and he is also shown with a gun in concept art, suggesting that he may be this class of pirate. Once Ratchet reaches Jasindu the corsairs gain red armour like the other space Pirates. They now have stronger attacks and are tougher to beat. In Ardolis, there is a skill point, called "Taste O' Yer Own Medicine", which can be obtained by defeating all corsairs in Ardolis using only the Combuster.