The problem is not the breed, but trying to integrate birds of such widely different age groups. Also, if your Marans are broody, they may see the new birds as threats. I would advise not to put young 2 month old chicks in with older birds regardless of breed as you are looking for trouble. You might try setting up a somewhat temporary pen within a pen, or a smaller pen in your big yard to put the younger ones in. Such that the older ones can see them and get used to their presence but can't get at them to hurt them. Make an entrance that the little ones can fit through but the bigger ones cannot, so after some time has passed (can't tell you how much time is needed but thinking weeks here) and you think you want to try integrating them, the young ones can seek safety from the older ones. This may or may not work but you could try. I just don't put younger birds in with older ones period until they are old and big enough to fend for themselves. I don't keep bantams, but I would think they should be kept separate. Some people are able to keep birds of all sizes together if there is a lot of space and a lot of stuff in the yard where they can hide, jump up high on, or in some way escape bullies. Then too, sometimes there is a bird that just picks on all the rest.
. I have a silkie roo who is great with my girls so I want to leave well enough alone! My Dusty roo is special needs, he almost died of wry neck as a 3 week old baby so I can't part with him or there would be no roo at all. So sad to have to part with Ivan as he is a beautiful and sweet bird. As is the case with all my flock family, I am very bonded to him and will break my heart to say goodbye. :'(
