IIRC, A hetzer is in the game files already. But it wasn't present on any of the current maps, so you won't see it yet.
Also, as had been said RO is an entirely different game on a different engine. In BF2, you cannot disable a tank by shooting certain parts of it, because it works off a hitpoint system. The best you can do is have certain parts of the tank take more damage than other parts.
The Haftholladung has been suggested before and, as has been shown, even made by a fan modeler. If that model is up to snuff, the FH2 Devs might incorporate it into the mod, or perhaps they will make their own version, who knows. If it was used in any sufficient quantity, you can rest assured that it will find its way into the mod one way or another (for example, the Haftholladung was in the Forgotten Hope Secret Weapons minimod for FH1).
Welcome to the forums. Don't lapse into noobishness and you should enjoy your stay here. Beware that the SS thing is a touchy subject, so don't dwell on it, at least not until you establish a decent reputation.