i keep my rcs in a 10 gallon. what i do is use a normal gravel vac, but i start it by sticking it deep into the gravel. i then barely pick it up each step and push the shrimp out of the way. i rarely get any shrimp, even babies this way. if i do i net them out of the bucket i siphon into after the water settles.
btw a good tip to remember is the larger the difference in height between your tank and the container you are siphoning into the greater the force of the suction will be. this is because the water is technically "falling" so it gains speed the farther it falls. keep it a foot or so difference and you'll suck up less shrimp, and the ones who do get in will swim right back out the tube. also, your tank will empty slower.