WARNING: RANT.
You know what I hate about HL modeling? It's illogical and overly complicated. Every model has different bones and submodels, and there's no way to turn one NPC into another without renaming bones. It should work like this for all models:
1. ONE body, no two bodies like in Op4, the backpack attachments go in a category below.
2. Heads. No limit, but all the same thing, just heads, no arms or anything.
3. Weapons. This includes barney's guns, the Hgrunt's weapons, and the scientist's needle.
4. Extra submodels/acessories. Op4 backpack and anything a mod's npc might need.
5. ALL THE SAME BONES FOR HUMAN MODELS!!!!
If I want an Op4 medic mesh on a scientist model, I should be able to do it without pain, if I want a Female assasin mesh on barney, I should be able to do it, and there should be a way to tell milkshape and other softwares to ignore bones in the meshes, just for things like the example directly above.
Now, doesn't that sound easier?
If every human used a common skeleton, they would all have to be the same size. You can turn one NPC into another by importing it's mesh onto the skeleton of another and reassign all of the vertices, though. It's pretty commonly done.
What is it that you're trying to do, anyway?
So far I've made a shotgun weilding massn model, a 'squadgrunt' hack, and a 'commandgrunt' hack, which I'm hoping to use in an op4 style mod someday, as friendly hgrunt variants.
Considering they're just hacks, I'd say they're OK, I'm using one in my custom models mod.