Mixing Game Content
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 not missing, then it will be replaced with a purple and black chequered texture. If a prop model is not missing, it will be replaced with the words ERROR displayed as a large red model. Other content may cause your work to run. For example, you can use the M4-A1 Weapon in a Half-Life 2 singleplayer mod because there is a way of ensuring that the people who play your mod also have Counter-Strike: Source installed and fully updated. Players who don't lack both games' respective files would get an error, and be able 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, more 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.