The game imposes a hard limit of -5 metabolic efficiency for xenogerm implantation. Any xenogerm below this threshold cannot be implanted. If your custom design pushes metabolism too low, you will need to add more negative genes to raise the efficiency back into the implantable range.
Explain how to these genes during a regular game. rimworld create xenotype
This is the most critical part of xenotype design. Your decision here will determine whether your custom species is a dying breed or a thriving race that conquers the RimWorld. The game imposes a hard limit of -5