This post will have lots of great ideas on how to quilt it.
http://www.quiltingboard.com/main-f1...t-t224144.html
As to the needle question, I've not quilted a t-shirt quilt on a longarm, but I've paid attention to those who have and made notes that I'd want to use a "topstitch" needle, size 16 or 18. They have a longer scarf, which gives the thread more space and reduces friction and wear on the thread as it moves through the fabric. But I could be wrong! Hopefully someone will come along and confirm or correct my info.