Half Life can handle any poly count for models, its just a matter of the persons computer. I've seen models that exceed ten thousand polygons before.
And just what am I going to google, Zombie? "Program that lets you convert Source models to Half life models"? Forget it, its much more convienient to ask people who probably already know where to get such a program then looking through thouands of search results, most fo which are probably other thread asking smeone for the same thing but then SOMEONE TELS THEM TO GOOGLE I AND THEY GET STUCK IN THE SAME THING. Wow, thats a mouthful.