There's a drastic difference between the ideal choice for a pirate 'Mech and what a pirate is most likely to have. Typically, a pirate uses whatever is available, not what he (or she) would WANT to use.
The ideal would be for something reasonably fast with hands, primarily energy weapon oriented, with some anti-personnel capability. Granted, not everything in the force would need to follow that format, but that would be the preferred "norm". An Ostsol or Ostroc as the commander's ride checks off the speed, hands, and energy boxes, and would likely have another 'Mech armed with machineguns or other anti-personnel weapons working in close support to cover the last base.
Most likely, a pirate is going to either be running something that he was operating when the unit he was a part of went rogue, or else something under-guarded that he was able to steal. Whether it was well suited for piracy or not will very likely have absolutely nothing to do with it. Whether the pirates are able to modify their equipment to better suit their needs, or have no choice but to modify their equipment (resorting to things even less suitable for the task) just to keep it running at all, depends on the situation and the abilities of the pirates.
If the pirate unit is operating with a "letter of marque", with the tacit approval or even direct undercover support of a House, then it may be able to acquire more of what it wants, rather than suffer with whatever it has. This obviously depends on the nature of their relationship to their patron, and the terms under which they are permitted to raid.
