You have lots of specific questions. But the one question you have in the back of your mind is “How do I make an awesome t-shirt that everyone will love?”. Hmmm … that’s tricky, but this should help:
The number one thing to keep in mind is your goal for the t-shirts. Before worrying about the details of design, budget, type of t-shirt, colors, sizes, etc. think about your end game. There is a dizzying amount of small but consequential decisions in a custom t-shirt project. Having your end goal in mind as a mental framework will make answers easier and quicker.
No matter what your intent is, a great golden rule is to make your t-shirt something people actually want to wear. People have too many t-shirts so if they’re grabbing for yours as soon as it comes out of the wash in 4 years this is a huge success.