Congrats! So, yes, they need a couple weeks to settle in, especially if they shipped.
And occasionally you do get a bitey hatchling. My very first ball python was a genuine biter. He grew out of it after the first year, and now he's very sweet.
But in the mean time: gloves. When in doubt, wear gloves. Especially when you're reaching into the tank to fetch the snake or to do a bit of spot cleaning. The bites are harmless, but they can sting, and they don't exactly build trust if you're not used to snakes. Gloves let the snake get used to you and your smell and handling while still letting you feel secure.
What they are thinking: Well, balling is basic fear. Most biting is a stress reaction. Defensive balling and biting is almost exclusively a hatchling thing among captive-bred ball pythons, because the adults are just so zen and used to handling.
Most hissing is because you just woke them up and they want to go back to sleep. Or you've dropped them in the past, and they don't trust you not to do it again.
If their tongue is flicking, they're starting to loosen up. If their tongue is flicking and they're looking around and periscoping, they're starting to have fun.