The way we use the Grim-N-Gritty rules is pretty much how Ken hood has them laid out with a few changes to the rules.
We use Vitality points ( basically your Hitpoints as per class and level) and then your Wound or Hit Points ( based upon your constitution with Ken's Progression table). Vitality represents the same thing as it does in every other system, superficial scratches, bruises, cramps, and wearing out.
Wounds/Hitpoints represent bloody gashes to the head, Broken tendons and such.
Critical hits go Directly to Wounds/HP
Sneak Attacks/Pounces go directly to Wounds/HP
OPTION 1: Sneak attacks could do regular +d6 of damage, half applied to Vitality, half applied to Wounds/HP. ( as in Forbidden Kingdoms)
A weapon with Penetration that exceeds the Protection of the target delivers damage directly to Wounds/HP.
Most, but not all, Called shots deliver damage to Wounds/HP.
I have not worked out how spells will work inthis mixed system.
All other attacks do damage to Vitality
We did have a problem with Monks. WE dont use Ken Hood's Martial Arts system though i suppose we Could. The monk has no way to penetrate heavy armour. We are in the process of figuring out a way for him to bypass Protection but still keep other Fighting Classes viable.
WE have several options:
Penetration Feats.
Replacing Stunning Attack with Penetrating blow.
Giving a Monk a Penetration Value equivalent to half his level.
WE play ina Low magic world. Bards are the nearest thing to Wizards. it has Worked nicely so far. The PCs have faced off agsint Elves, Kobolds and Ogres. 4 Fourth level characters could not take out even one ogre in 5 rounds of combat before the ogres stole some captive elves and ran into the woods.
Rule#1 Do not pit your Characters against anything with High Protection values until much later in Levels!
We also dole out many more feats than would otherwise occur, particularly for the fighter.
This is to represent that they are the pinnacle of the warrior elite in thier society and they have been trained their whole lives to Explore the Forbidden World (and i also just bought the Mongoose book with all those feats in it).