You can't take a HL2 model in HL1, unless you convert it back to a HL1 player model.
If your new player model is called santa_claus, then the make a new folder in your valve/models/player with the same name as the model file. In this case: valve/models/player/santa_claus/santa_claus.mdl.
Only then the player model will show up in your "select player model" screen in your Steam settings. Im not sure what it is called.