Or just give it a fairly low percentage check pass rate, like a percentage for every point above Charisma 10. So, with a Ch of 11 you'll have a 1% chance of being able to offer peace when 'C'hatting to a hostile. Also let it scale with experience level and danger level of the monster minus experience level of the PC.
Edit - Maybe even with very high Ch you can talk a monster into becoming your companion.