Though now I only have bantams (less space and food required) I used to have both, and I never had a problem other than initial squabbles. Yes, in my experience raising them together is MUCH better, they sort out their issues before anyone is big enough to hurt anyone else. Growing up with the same pecking order is better than making them figure out a new one by introducing other adult birds. Some bantam breeds are mild by nature and might not be such a good choice for mixing though. Examples of bantams from personal experience are listed below.
Feisty:
Old English
Seabrights
Ameraucanas
Rosecombs
Hamburgs
Timid/Calm:
Cochins
Silkies
Sultans
Faverolles
Polish that can't see
Feisty:
Old English
Seabrights
Ameraucanas
Rosecombs
Hamburgs
Timid/Calm:
Cochins
Silkies
Sultans
Faverolles
Polish that can't see