I am sorry you cuckoo marans didn't do well with the chicks....
All flocks (like all individual birds) behave differently towards chicks, some are great with them, sometimes members only tolerate them because they don't want to upset the broody and some can be hostile toward the new ones.... the broody will usually follow her instinct as far as the amount she allows interaction, though careful observance by us can help greatly, especially with a new broody. I tend to agree with Aart, and let the broody decide, but allowing them a few days to become more mobile before intermingling is not a bad idea when you have a flock you aren't sure of, or even one or two flock members who are iffy. My flock tends to be very broody and chick friendly, but I still make sure there are plenty of places for the new family to hide if need be and I constantly watch for any changes in behavior with the adult flock members.
Remember that flock dynamics are a fluid thing, the attitudes and positions of the birds ebb and flow constantly.... a hen who was friendly and interactive with the broody family when she was high on the pecking order may suddenly be less friendly to outright mean if her flock position changed for some reason because she may suddenly decide the new additions are a further threat to her status....
I like brooding within the flock, and over time have established a group that is very low threat to the new chicks, but any time you raise chicks in the coop there are inherent risks that you have to accept, accidents can happen, aggressive behaviors can sometimes crop up and chicks can get separated from mama and get into trouble.... to avoid the risks means you would have to totally separate the broody and chicks... for some people it works, for some it isn't feasible and for some it isn't desired for other reasons. Weigh your options (think about the rarity or value of your chicks if it was a long awaited egg source) and what you consider your 'acceptable risks' and go from there.
