) So you don't know if you are keeping a fish in the beginning of it's life, mid life or in the end stages of it's life. This is why you shouldn't get too upset when you lose a fish when you've done everything else right. It just happens. As my Vet once told me when I was bringing in my dog to be put out of her pain, " Sadly, we humans keep pets that do not have a lifespan as long as our own so we are going to be experiencing deaths for every pet we keep. The best we can do is keep them as healthy as possible while we have them."
There is a distinct size difference between male and female cories of any species so not all small ones will get really large. You need to be careful about adding fish to your tank when you don't see them beforehand. That's the easiest way to introduce diseases into your system. This is why you use a quarantine tank for an extended period. Cory cats are best purchased in schools and not as individuals as the singles can freak out being alone and not all cory species get along together. In schools there is safety in numbers.
I hope I am not overstocked, not again. I hope. 
You have to be careful with sites like that. A lot of their information for the fish is based on wild fish and you more than likely DO NOT have wild caught fish. It's more important to match the water your fish are coming from and not what the internet says. As for capacity, again, you have to take into consideration where in the tank the fish are swimming. You could be under stocked in total fish but if they all are swimming in the same area, that's not good. 21 cory cats in a 40 high is a lot of cories. They don't care about any water above 6" +/- from the bottom. If it were me, I'd cancel the Julii and give the tinys to the pet shop or back to the pet shop.
You have to be careful with sites like that. A lot of their information for the fish is based on wild fish and you more than likely DO NOT have wild caught fish. It's more important to match the water your fish are coming from and not what the internet says. As for capacity, again, you have to take into consideration where in the tank the fish are swimming. You could be under stocked in total fish but if they all are swimming in the same area, that's not good. 21 cory cats in a 40 high is a lot of cories. They don't care about any water above 6" +/- from the bottom. If it were me, I'd cancel the Julii and give the tinys to the pet shop or back to the pet shop.
You said you weren't going to get anymore of the glo tetras and yet you did it. What's wrong with that picture?
I really love my Glo tetras. It should work this time. but if not. then I'm done. I really loved my guppies, but I can not have them because of the tank mates I have, that would not be good. I already ordered the Julii fish and their medium seized. I also ordered my Glo Tetras. They are on their way. I would like to keep these fish, so I dare to ask how much water changes would I have to do to keep all these fish?
It's going to be more how often than how much. You are really going to have to keep testing your water to see how quickly your nitrates are rising and adjust your water changes accordingly. There is no formula that says "If I have this many fish I need to change that much water." I foresee you are really going to need to adjust your water for the changes so that you are not continually adding ammonia/ammonium when you do the water changes. That will reduce the amount of conversion that goes on during your current water changes. This is going to need to be done in another container with ammonia removers not ammonia detoxifiers. ( This is the price you pay for overloading an aquarium.I know I said I did not want anymore Glo Tetras. but I changed my mind.
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If you recall, the OP was doing frequent water changes to combat nitrate rise which was not reducing her nitrates for long because she was constantly adding ammonia ( via the neutralized chloramine). Now with too many fish in the tank, more frequent water changes are going to become more likely necessary which will start the whole circle starting again. By adding water with 0 ammonia/ammonium, it extends the time between water changes.
) When chloramine was first used, we hobbyists had to adjust to using new methods to make the water safe for the fish. In this case, another change needs to be done to make the water safer for her overloaded tank. Yes, it's more work but not an insurmountable task.
You're only adding the level from the water change. What about the amount of ammonia all the fish and foods will be adding?
