The picture above is probably about as far as i'd ever go with age differences, I reckon the older ones are 3-4 weeks and the small ones just a few days old.
I think the other picture had quail and partridge as well as young chicks and maybe ducklings?
I wouldn't say I never see any fighting but no more so than between groups of similar aged birds, the smaller ones have plenty of room to back off if they need to and usually its no more than a quick peck on the head if they get to close to one another.
Quail are the problem, immature quail of 4 weeks or more will kill new quail chicks very quickly so I need to keep them fairly well separated out by age/size.
The rest seem to be fine together, I have two week old turkeys in there just now and the odd one of them can be a bit of a bully but no disasters so far.
Another factor may be that my brooding areas are always pretty busy, if there were just a small number of different aged birds in them there may well be more problems with the small ones being chased and bullied but when its busy they seem to tolerate each other a lot more.
This picture has week old turkeys, Scots Greys roughly 0-2 weeks and quail about 2-4 weeks that I can see, maybe more