Yes, you can eat them(tilapia) and they are generally (depending on what they have been eating) about the best fresh water fish to eat.
No, grass carp(white amur) are different species and survive year around in most ponds. They can serve a useful weed control function that complements Tilapia very well. Personnally, I stock very small numbers of grass carp in all my ponds for weed prevention/control. They can be very effective on certain undesirable water plants. Several threads on here discussing them.
