Are you talking about the true dwarves? They can harm fish, but it's really rare. If they can find a sick, dying, or dead fish, they will definitely take advantage. Your cray will enjoy all of the hiding spots.
I'm using root tabs. I moved it recently. It was in the shadow of a regular sword and was not in the best shape. Now, it's starting to put out lots of new growth from within. I believe I will cut off the outer leaves that don't look healthy.
I work in a water resource lab. The next time I change the cuttlebone in my tank, I'm going to see what results I may get in some of our common tests concerning that outer gross and stink of an old cuttlebone.
I didn't realize Frogbit grew as fast as it does. This picture was taken today, two weeks since I thinned it out so that there was only 1/3 of the water covered. I'm glad it's sucks up Nitrates quickly.
Is the cray that you have a regular sized cray? If so, it would probably eat a dwarf cray. Also, the good thing about the dwarf crays is that they are not strictly nocturnal so you will see them out and about throughout the day.