Chickens do not need additional heat in those types of temperatures after they are fully feathered out. Most chicks are fully feathered out around 4 to 5 weeks old. 12 week old chicks will have absolutely no problem with those temperatures.
You need to communicate directly with My Pet Chicken. We can guess what their policies might be but they will know what their policies are for your area. I suspect they will require you to order a minimum of 5 but a minimum of 8 is a real possibility. But I am guessing. They will know.
At 12 weeks old when you integrate, I feed grower to all with oyster shell on the side. Grower has the correct amount of protein for layers and 12 week old chicks. Other things work too.
I have successfully integrated 12 week old chicks (roosters and pullets) with a flock of grown hens and a grown rooster, but I have quite a bit of room. I housed them side by side from the day I hatched them, keeping my brooder in the coop. When the chicks were 4 weeks old and fully feathered out, I moved the adults to a separate coop where hey could see the old coop but not get into the coop and run. The chicks got the old coop and run. I let the adults free range but kept the chicks locked in the coop and run until they were about 8 weeks old. Then I alternated the free ranging, letting the adults out one day and the chicks the next. When the chicks were about 10 weeks old, I let them free range together. They slept in separate coops. At 11 weeks, I could put the adults in the same coop but opened it up early in the morning before they came down off the roosts. At 12 weeks, I could safely leave all of them in locked in the coop and run together. I did provide separate food and water for them because the older ones will bully the young ones and keep them from food and water if they can. By about 16 weeks they share food and water.
Integration can be a challenge. It is my most worrying aspect of raising a new batch of chicks. All our set-ups are different so different things work for different people. I think the keys are letting them get to know each other before they can get to each other, giving the young ones room or a way to get away from the older ones, and providing separate food and water. I think another key is to not interfere with them as long as no blood is drawn. It is natural that they need to set the pecking order themselves.
Good luck!