Absolutely, even blankets, sheets, fleece, flannel will work as batting. They work well because they have already shrunk up (except raw flannel ;)).
However, big thought: They will be heavy quilts, suitable more for tying than for quilting.
My first quilt used an old blanket as the batting. I tied it (didn't know about Machine quilting back then). It is still in great shape (seams popping but fabric in great shape), but so heavy I hardly use it.
The loftier the comforter, the harder to quilt.
How do you plan to finish the quilt top (lots of quilting, tied, pillowcase style like a duvet cover???) That will make the difference.
Show us and we can help you decide.