Yes, you are correct. As you say, it is a common mistake that is made.
With respect to the original question the size battery charger that suits ones particular needs depends so much on how much time is available to charge the batteries, how depleted they normally get between charges and what other DC loads one has running during charging (ie if one is drawing 20 A for other DC loads while charging then the charger will have to meet that demand too so reducing the rate of charge to the batteries) that it is not possible to give specific advice based just on battery capacity - except as others have done and suggest the 10% guideline.
If the battery capacity is actually 300 odd amp hours (ie there has been a mistake in working out the capacity of the series/paralleled batteries) then the existing 50 A charger will almost always be plenty. If the battery capacity is actually 600 odd amp hours then unless one has some particularly short charging time needs or a need to run other big DC loads while charging then I would be inclined to give the existing 50 A charger a trial to see if it meets your needs (assuming it is in ok condition and there is no other pressing need to replace it now).
We find that with 520 AHr of wet lead acid deep cycle batteries a 50 A charger is plenty for our particular needs (so meets the guideline of around 10% of capacity that others have mentioned) and still would be so with an over 600AHr bank of batteries. That given our own particular usage profile where most of our deep charging is done away from the dock and shore power, so main DC loads in the dock are the refrigerator, freezer, TV, lighting, etc rather than deep charging.