First, a 300 BAR regulator can fit either a 200 or 300 BAR valve, but a 200 BAR regulator will ONLY fit a 200 BAR valve. If your 120 is truly a HP cylinder, it will have a 300 BAR valve on it, so bottom-line, get a 300 BAR regulator. If your shop was unable to answer this question for you, you're possibly in for a world of hurt.
Second, traditionally HPs aren't overfilled, and even if they are, you don't get nearly the amount of benefit you get from overfilling LPs. Ultimately HPs aren't overfilled because they fall into the "why bother?" range of benefit.
Third, it's your next statement that leads me to believe that there's some confusion on the part of the dive shop:
"...the guy at the dive shop was telling about overfilling the tank during the 1st hydro period."
This can be taken several ways, all which are incorrect.
The guy could have been talking about the plus rating. This would make him twice wrong, since HP cylinders are not + rated to begin with, and for LP cylinders that ARE + rated, they can be + rated for their entire lifetimes, not just for the first hydro period. The latter is a very, very common misconception that many dive shops have.
Or he could be saying that they only like to truly overfill during the first hydro period. I know LP cylinders that have had several hydros that are merrily being overfilled today, so again even on that level the statement doesn't make any sense.
Be very, very careful when getting steel cylinder information from dive shops. Many have all sorts of misinformation that they dish out constantly.
Hope this helps.
Roak