1: "Turbo" snails mostly are my advice, nassarius if you have a sand bed as they'll burrow and when you feed their proboscis(?) will come up like a submarine's scope and then go searching for food. If you have a 245g tank, get the amount recommended for a 25-30g tank the amount of "cleaning" does not increase linearly with size (and they oversize the "recommended" amount too), and your snails wont starve to death if you buy less.
2: None, if you want crabs to have crabs great, not necessary for clean up, if you feed that much food that it settles on the bottom you're feeding too much.
3: Shrimp don't clean up, except in the way crabs do, so treat them like crabs get them if you want shrimp but not for a clean up crew. IF you spot feed LPS they will yank the food out of them.
If you're going with a sandbed, a cucumber or two might be nice, it's fun watching them eat dirty sand and poop out clean sand, but again don't go by what is "recommended", I literally would cut that down by a factor of 10