There's a nice little best practices of what IDs to avoid besides 5C
here.
Basically avoid using 5C and the following:
- FG - Frog God Games
- GR - Green Ronin Publishing
- HG - Hammerdog Games
- KP - Kobold Press
- LG - Legendary Games
- RP - Rite Publishing
This helps you avoid conflicting with the deployments, both the community and the official content markets.