The size being 45" in length and width? That's probably big enough for brooding 8 chicks to about age 5 weeks or so (which is around the time they are fully feathered.)
Common space guidelines for chickens:
1 square foot of space per chick up to about age 4 weeks (can be a little less in the first week or so)
2 square feet of space per chick from age 4 weeks to 8 weeks
4 square feet of space per chicken from age 8 weeks on through the rest of their lives.
That 4 square feet of space would usually be in the coop (where they sleep and lay eggs, and hang out in bad weather.)
Usual advice is to also have at least 10 square feet of space per chicken in an outdoor run, where they can spend time in good weather.
Plus 1 linear foot of roosting space per adult chicken, and 1 nestbox for about 4 hens (so 8 hens would have 2 or maybe 3 nestboxes. They definitely will not need 8 nestboxes unless they all go broody and try to hatch eggs at the same time.)
It looks like that pen has between 11 and 12 square feet of space (I found an online calculator and plugged in the numbers.) That is more than enough for 8 chicks up to 4 weeks old, so you may be able to go a week or so longer. They do not really need their space doubled on a particular day, despite the way space recommendations are usually phrased.
Right at first, chicks naturally try to go under things to stay warm. I am not sure how they would act with a heat source underneath them at that point. Once they are a bit older, that would probably work (based on the stories I see of chicks that like to hang out on top of a brooder plate, which would also be warm under their feet.)
You might consider something like the idea in this thread:
https://www.backyardchickens.com/th...d-in-the-brooder-picture-heavy-update.956958/
It is a way to let chicks go underneath a heating pad, similar to how they would go underneath a broody hen or a purchased brooder plate. I have no personal experience with it, but apparently quite a few people find that it works well for raising chicks. Maybe it would work with one of your reptile mats?
(And I see that someone else recommended a mama heating pad while I was typing this!)
Yes, plenty of people do that.
Chicks do like to scratch around in their bedding (like shavings), but they can be okay without that until they move outside.