Sorry, hectic day! I don't actually agree that apple snails are hardier than fish. Alot of them do quite poorly with ammonia and nitrites, their symptoms are just less visible as they can breath with their siphon as well. I would totally go for a 20g set up with some nice low light plants. Neocaridina shrimp are by far the easiest as far as water conditions go. They accept a very wide range of ph and temp. Caridina can be much more particular about both if you want them to breed. Apple snails (pomacea bridgesii) are by far my kids hands down favorite. They love the shrimp too, and in an invert only tank you can expect to see lots of action. The bioload of shrimp is very low, but apple snails are "poop factories". I would recommend getting your tank set up with plants and preferably cycling it before you add your critters. As far as substrate, the shrimp dont care, but the snails don't always love sand. I would go with a darker substrate as I feel like the coloring on shrimp (neocaridina anyway) is better with a darker substrate. Definately get some java moss for the shrimp. The baby shrimp love it. Filtration is super important in a tank with snails, so over do it if you can. Please be sure to place a sponge over the intake to protect both the snails and any baby shrimp you may have. Temp for these critters is better in the low to mid 70's. Higher temps accelerate the apple snails growth and shorten the life span. I keep my invert tanks around 74. The snails will need a calcium rich diet (often accomplished through feeding veggies) and the shrimp are pretty opportunistic feeders. Hope all this helps, let me know if I forgot anything important 
