How far apart in age is it safe to mix ducklings?

[newbie duck momma]

Hello,

I went to the co-op yesterday to get some food and a heat pad for my upcoming ducklings that are on day 24 in the incubator, and they had just gotten in baby ducklings. Im a sucker for the ducklings and have never incubated before and am worried I will only get one of the three to hatch or worse zero. The eggs were not prepped to incubate, and honestly I wasnt planning to get any ducklings this spring, but I lost all four of my feathered babies in a fire. I had about a dozen eggs and last minute decided to incubate the eggs, which I have never incubated anything. My question is, I have 4 2 day old ducklings now, 2 blue swedish, and two buff ducklings and three in the incubator at day 24. How far apart in age is it safe to put ducklings together? I know I shouldnt have jumped the gun and am prepared to have two nurseries set up, but would like to be able to put them together at some point.
Heres a pic of the two day olds:

 

[CoriM]

Sweet! I have mixed ducks that were a week apart with just a few hours of close watching to make sure the big ones don't bulley. I currently have runners and a buff that are two weeks older than two black swedish, so 4.5 and 2.5 weeks old. I haven't mixed them yet - the size difference was enormous. I am now at the point where I am letting them be together outside in a big run when I am with them to supervise. They sleep in separate brooders.