Do I need a certain needle to do FMQ on a T-shirt quilt? If so, do I only use that on the t-shirt part and regular quilting needle on the cotton parts? I only plan to go around some of the characters, not heavily quilted in the T-shirt blocks.

Hmmm, good question. I use a ball point needle when sewing jersey normally. Don't know if you would need to use one when quilting the T Shirts. A sharp needle actually cuts the threads in the knit and you can get holes and runs. A ball point needle sort of nudges the threads in the knit apart.

I used a 90 or 100 when I made a T-shirt quilt. Make sure you have stabilized the t-shirt block (can be found in LQS or Joanns') so it doesn't stretch when you FMQ it.

90 or 100 Quilting type needle?

I used the Tricot stabilizer, with the stretch in the opposite direction of the stretch in the t-shirt. It's very nice feeling! I am adding some borders to the blocks, then trimming all to the same size before adding sashing and borders. I'm excited to see it done!

I would definitely use a ball point needle on anything knit, especially a finer knit like a tshirt.