Its just a needle. 21g is no different than a 22g-as in you wouldnt be able to tell the difference.
Draw with a 3cc 20-22g, and shoot glutes with a 23g 1.5" and all other sites with a 25g 1".
Days for injects dont matter as long as you keep to the half life. The others here have answered very well to all your questions.
Good luck on your first run. And btw-no need to rush buying the pins. Theres always more reading to do so just get to the cycle when you get to it-its not like you have to prep for a bb comp.