I would not worry about quarantining chicks from the different major hatcheries. One of them had a disease problem a few years ago, but they took care of that. A hatchery would go out of business real quick if it consistently shipped diseased birds. The chicks at the hatcheries do not come into contact with older chickens so they do not get infected by older birds. That incident a few years back shows how fast disease can spread in a flock, but the hatcheries take biosecurity pretty seriously. I don't know what happened that time a few years ago, but I would not hesitate to mix chicks from different major hatcheries. There is always a chance that something could go wrong, but I think the chances from recognized major hatcheries is extremely small.
There are a few diseases that can be transmitted by the eggs, but very few. Many people have their flocks NPIP certified which means they regularly test for certain diseases. To me, the safest way from a biosecurity perspective is to hatch NPIP certified eggs. You can arrange for shipping of eggs from various breeders to arrive about the same time to go in the incubator. Of course, you never know how many will hatch or what sex they will be, so this may not be your best way to go. There are tradeoffs in everything.
I have not gotten chicks from MPC so I cannot comment on their quality as you describe it, which I take to mean healthy animals that pretty much look like the breed is supposed to look like. I don't know where they get their hatching eggs. I suspect it is from the same sources some other hatcheries get their hatching eggs, so they may not be any different from some other hatchery chicks. They appear to be a quality organization and their prices reflect that. I don't know how many chicks you are after, but MPC may be your best bet for the breeds and sexes you are looking for.
