Small update. While using merge-rrd.py (as I was having trouble with the rrd-merger.pl option) I have found that the Quality graphs merged successfully in a test and display properly.
System, Traffic and Packets however produce the "There has been an error creating the graphs. Please check your systemlogs for further details." message.
Further digging reveals that these are of different data structures. I will mess further, but found a tool that takes into account different data structures:http://oss.oetiker.ch/rrdtool-trac/browser/trunk/contrib/rrdjig
And I forgot there is a more up to date merging tool available here:http://code.google.com/p/merge-rrds/
It merges XML as well as RRD
Just haven't tried it yet.