Is there a way to retain the stock rank/achievements/custom classes so that players can use my mod without having to start from square one?

I'm working on a custom mod but don't want players to have to re-rank and lose all their custom classes. I've looked around _rank.gsc, but not really knowing what to do.

yes and no...

the ranks are related to the mod, if you change the name you have new ranks... to avoid that is mod the original files at the main folder, and this I would only recommend if is something local like in a LAN or something, not online, because might ban you for cheat (but maybe not since PB is dead for COD4)

or if your mod is simple and could be server side, you can put the files just at the server and will work.

if not, you will have new ranks to your mod or make without ranks, like Mod Warfare.

cheers