Short answer: You are getting ahead of yourself when it comes to adding NPCs.
Long answer:
The data in the FGD is seperate from the game's code. Just because there's a reference to the entity in the FGD doesn't mean it's coded into the game, and just because it's coded into the game doesn't necessarily mean it's referenced in the FGD. To introduce new code into the game will require at least a rudimentary knowledge of C++ and analysis of the existing code. We have a few tutorials here that can help you along your way for that.
So, to get monster_zombie_barney and monster_zombie_grunt into your maps, you'll need to do one of three things, all of which require making your map and/or mappack into a full mod:
1) Use the Op4 DLL files, which will include code for all Op4 entities. Use of Op4 is needed, extra code is not possible.
2) Use the HL SDK code to make your own mod DLL that includes copies of monster_zombie using the desired models/sounds/etc. Coding is needed.
3) Use
Spirit of Half-Life, (Backup/download links courtesy of our own Penguinboy) and then use regular monster_zombies with the model field set to the desired models. Coding is optional, but possible.