In principle, your approach using itertools.groupby, the existing _sum, your decimalsum, and my _ExactRatio class could be combined.

You could call decimalsum for Decimal and subclasses, the current _sum for everything else.
_sum would return an _ExactRatio instance now anyway, just modify decimalsum accordingly and you are done.

The problem I see is that it would cause a slow down in many cases where no Decimals or just a few are involved (think of mixes of ints and floats as a realistic scenario, and consider also that you would have to do an isinstance check to catch subclasses of Decimal).