It's my bet that cinder blocks (the kind with the big holes through them) would be an excellent choice, even superior to rocks. They maximize surface area for small biological growth and maximize cavities for the fish to hang out in.

There might be some pH concerns to consider depending on the water chemistry, but that would also be the case with natural rock depending on the kind. Minimal concern I would think, but worth researching.

Natural rock would be better for crawdads, however...imo.

I have to say...it depends, regarding the cinder blocks to some degree. If you want recruitment of your stocked fish, the added hidey holes will be good.

Fish on!,
Noel