I have a Stag 8L that I thought about converting to a pistol.

1. I have heard that if it was sold as a rifle you can't convert to pistol because the serial number says it was sold as a rifle. Is this a myth?

2. are there gas piston systems for AR pistols or is it possible to convert it to a standard gas system?

You can't convert a rifle to a pistol unless it was first a Pistol
Yes, there are pistol length pistons
Yes, you should be able to convert to DI

ETA: Maybe best to just purchase a Pistol lower, swap uppers, and build new upper for Rifle lower?

You can't make a rifle into a pistol unless it was pistol first, once built first as a rifle always a rifle. Your only option is to make it an NFA item and SBR it. If you want a pistol you are most likely going to have to start over with a brand new lower.