Even at this early age, it can be dangerous introducing different birds. In the picture below you can see a Rouen that appears to be leaning against the stock tank wall, but it was actually pushed down by a naughty Freedom Ranger who did not take kindly to all the duck immigrants. In fact, he had to be watched closely because he kept running around latching onto duck flippers with his beak and dragging ducklings through the shavings as if he were Achilles.
The other day there was a certain El Pollo Diablo that just kept pecking at everyone. He was placed in a stock tank by himself and peeped mournfully, refusing to eat and drink. We took pity on him and returned him to his tank--he drank lots and lots of water then resumed his pecking.
With adult birds you can introduce immigrants at night then see what happens at daylight, but with chicks, it can be a bit more tricky since they have the light on all the time and just run around whenever they want. I have never had many problems, even with overcrowding; you are more likely to see problems in stores where 1,000 birds arrive at the same time and customers are frantically called.