Pulled straight from the Valve Developer community, link
here
"Mixing Game Content
From Valve Developer Community
It is not advisable to mix game content - use weapons/materials/models/maps from different Source games - if you wish to run your work on all players’ machines. This is because there is no guarantee that the person downloading your work has installed all the games required, and so there is no guarantee that your prospective player has all the materials/etc required installed.
If a material is missing, then it will be replaced with a purple and black chequered texture. If a prop model is missing, it will be replaced with the words ERROR displayed as a large red model. Other missing content may cause your work to not run at all. For example, you can not use the M4-A1 Weapon in a Half-Life 2 singleplayer mod because there is no way of ensuring that the people who play your mod also have Counter-Strike: Source installed and fully updated. Players who lack both games' respective files would get an error, and be unable to play your mod.
This is especially important when creating content which will be auto-downloaded from a multiplayer game server, or creating content for existing mods (e.g. a Counter-Strike map). It is, of course, less important when creating a single-player mod, since it will be possible to warn a player ahead of time what the requirements will be for running your mod.
Note: It is not acceptable to extract content from GCF files and redistribute it with your work as a way of getting round this restriction.
This violates the Steam EULA."
I'm not bitching about what you are doing, just pointing you to the official word.
Word.