What a cute panel!
Here I my thoughts on this...I'd cut them apart, arrange them to my pleasing, and add at least an inch of sashing between, making them into a center panel in the size and shape I wanted. Then I'd add some borders, maybe a solid color one first, and some kind of wonky blocks in matching colors... How many and what kind of borders you add depends on how big you want the finished quilt. But, I'd get the middle section arranged first, and go from there.
If you're using fleece for backing, you probably won't need any wadding (batting) in addition to that. And, yes, I would quilt right through both the fleece and the top after they are together.