I have had multiple cuckoo marans - and I agree they aren't as laid back as the BCMs I have had - I have never found them to be bullies. Not quite as large as my BCM (but still good sized), a bit more 'active' within the flock (i.e. a bit more assertive), but none have been bullies - not in any way. Granted, I have only had a few (3 100% Cuckoo marans, 1 which was a rooster - a GOOD rooster), and I have had multiple that were mixes - offspring from either the hens or the roo...with someone else. Basically, barnyard mutts what were wonderfully diverse and great hens in their own right.
All I can say is: each chicken has it's own personality - but my experience is that cuckoo are friendly, intelligent, and 'in command', but in no way bullies/meanies.