If you are trying to avoid shrinkage, the flannel will need to be prewashed. Some folks even say to wash flannel more than once to get all the shrinking done before using it as batting.
I don't find Warm and Natural to be overly hot, but in the desert it might be too much for you. As for the shrinkage on W&N, it gives the quilt a soft crinkled "antique" look which I love. Some people do not care for that look at all. It's a personal preference kind of think. W&N can be prewashed, also, so it won't crinkle your quilt.