The different ages of the individual eggs shouldn't matter. As long as they are put in the incubator at the same time(unless you want a staggered hatch, but I doubt you do). You should be able to see the air cell, especially if they are more than a few days old. The air cell is always there, even if the eggs aren't getting incubated. It's just very small at first. For me, I think that 55% humidity is too high. I keep mine at 30-40 for the first 18 days, then I up it to 65-80%. But that's just me. And yes it's like the same thing(doing your 'thing' with the eggs and the mother getting off the nest). I candled all 9 of my eggs from day one, and two weren't fertile, and one quit, and I got 6 to hatch. This was my second time hatching with an incubator. I don't think candling often hurts the embryo or the development of it. And the temps jumping down a bit isn't too much of a problem. But it jumping up is. For this last hatch, on day 3(day 0 for one egg..Kinda did a bit of a staggered hatch..Milo hatched just fine by the way) I went out with some friends. When I got home about 5 hours later, the incubator was off, and the temp was 74 degrees. I;m guessing it was off the entire time I was gone. The wall socket tripped or something like that and I had to reset it. I thought that the embryos might have gotten too cold and died, but they were fine. And I don't weigh my eggs, I just candle to see the development of the air cell(but obviously you can do whatever you prefer.I was just stating what I do). If most of the air cells are too small, humidity goes down. If most of the air cells are too big, humidity goes up. In this last hatch, I had a bantam egg and it's air cell was tiny, and then in the same hatch, I had a large fowl egg, and it's air cell was too big. But all the other eggs' air cells were fine, so I left the humidity, and both of those eggs(and the other eggs) hatched just fine. So don't panic(like I did lol) if the eggs aren't exactly the same. If the majority are fine, then leave what you have working. And btw, it's really fun to candle the eggs to watch the little embryos swimming around, seeing their hearts beat, and then later seeing them press their little feet on the side of the egg...Have fun hatching, and I hope you have a good hatch.