Now you're asking me to start thinking.
I use a
Vergez Blanchard #43 diamond awl for most of my work. It's the smallest available but makes a large enough hole for most applications (two or three layers of 8-9oz). For really thick leather, I have a longer awl I made which will punch through more layers of leather.
For needles, I have
#4's & #2's . I don't recall exactly which size is which but I just take the smallest for the thread I'm using. Remember that you can put thread through a smaller hole than the thread diameter once it's waxed. One thing I was taught years ago was to wax the thread toward the midpoint from both ends so it lays the fibers down so the thread doesn't pull against the hole when running the thread through the leather. This also makes it easier to pull the thread through the needle eye.
I wish I could be a little more helpful. Maybe one of the pros on the forum will be able to advise you better.