FWIW, I have two STI Trojans. I have pestered the guys at STI factory on variety of topics, and fitting a slide stop is one of them. I asked if I could buy an extended slide stop and just drop it in mine and he said their slide stops are actually fitted to the gun. I believe there are different diameter pins available for fitting down. You may want to call STI and ask them about it.
FWIW: if the stop can fall out after the gun is assembled it probably means something is wrong in the spring tube assembly like the plunger on the muzzle-nearest end is jammed in and not springing out to push against the stop. You can bore a small detent indentation for that ball to snap into in the face of the stop to keep it from walking out, but it should not need it to stay in.