Even though there are a lot of folks that use "generic fit" with their SERPA holsters, I'd not do that with a holster that locks in the trigger guard. There is some slop in the SERPA since the retention is provided by the lock rather than the shape of the holster, so you can often put a gun not designed for your SERPA in your holster, but I'd get a SERPA designed specifically for your gun.
Here's my unsolicited SERPA rant. First off, I always assume all SERPA users are safe and properly trained in the operation of their holster, so I'm taking the "shoot yourself" issue off the table.
For most holster users, they are looking for either a concealment holster or a range/training/competition holster. The SERPA is a poor concealment holster as the gun is held away from the body, and the grip is not pulled to the body. For a range/training/competition use, why would you want a lock on your holster? Those you are training/competing with typically won't have a lock on their holster and you'd be at a disadvantage.
Certainly, there are places where the SERPA is a reasonable choice, ATV riding, hunting, fishing, some military applications, or even open carry. However, if you need a lock on your holster you're probably better off with a Safariland holster.