I am planning on getting a shrimpaquarium and I wonder if crabs can be held in the same aquarium as shrimps?

In my dreams I would want to have a 30-40 litres (8-10 US gal.) aquarium only half filled so the crab can breathe. With some plants for the shrimps and a root with a piece of cork connected with it for the crab so it can climb above the surface.

I have also concidered to get some snails to keep the algae away but I donīt know if they can be combined with shrimps and crabs.

So my questions are:
# Is it posible to keep shrimps, snails and a crab in the same aquarium?
# Will the crab concider the shrimps and snails as food?
# If the shrimps gets "children" will the crab eat them too?
# If it is posible to combine any of these species. Which sort of crab, snails and shrimps should I buy?

I hope that someone knows the answer to these guestions and takes the time to answer them.

Best regards Alfred