Definitely don't recommend mixing start dates. Just incubate twice - more fun anyway
Last thing you need is a chick spending days on its own.
Posted eggs can do well but often produce poor results, especially if packaged in the currently popular styrofoam type egg boxes - they have nowhere near enough padding, and it's the padding / stuffing in the box (the way Amazon for example fill excess space in their boxes with paper and bubblewrap) that provides protection against shocks. So that's the first thing to watch out for with online sellers: how they package the eggs and are they willing to add better protection.
The second thing is to stay clear from are sellers who claim they can't be held responsible for poor hatching rates when eggs have been through the postal system because that affects fertility. It's nonsense, being posted affects
hatch-ability (as per the above), not
fertility, and you can check for fertility even when the non-developing eggs have spent that first week in the incubator already.
When you get the eggs, candle them already and see if the contents sloshes about or not. Check against one of your own chickens' eggs if you're not sure. If the egg yolk moves all around the egg when you turn the egg it's a bad sign, it's been treated too roughly (e.g. the postman's bag bumping all the time during his round), and leaving it rounded end upwards in an egg box for 24h as mentioned by other people helps to hopefully resettle everything so that the air sac will be in the right position.
Will all the eggs come from the same seller? If so, then I would order 6 eggs of one breed, incubate them, and only if fertility was good I would order the other 6 from that person. Remember that you're unlikely to have a 100% hatch and there could be many boys.
Something tells me you'll be googling lots in the coming weeks
Have fun!