I have a 10 gal nano reef (invert tank)
it is very challenging, especially if you have no experience. I've learned patience. LOTS of it. The water you have to test nearly every day (I do mine 3x a week) and refill the evaporated water at least once a week and frequent water changes.
There isnt much of fish you could get (I risked and got a few) and I hear there are plenty of corals you could get that are small and hardy. just do research before adding any.
As for crabs and shrimps, mine does great in a 10 gal. I have 2 shrimps, and aiming for a sexy shrimp if I can ever find them. 3 hermit crabs and a porclian anenome crab are doing wonderful too (After months after cycling and killing.. oops.)