If this were my tank, and I have had a few tanks under 5g I would go with this:
2 nacrissus snails (spelling is wrong, i know lol)
1-2 nerites or something similar
PLUS some shrimp....
either a fire shrimp or a cleaner shrimp. OR you can go with 3 sexy shrimp. they are awesome. Tiny, but they dance a sexy dance.
Crabs: I would skip the usual hermits and maybe get a strawberry crab, pom pom crab or maybe an emerald crab.
Now for fish, which is the big topic here. I would, and have, kept clown gobies in a 4g tank for quite some time so I think that would be your best bet. I like the green ones and the yellow ones. They will swim a little bit here and there but usually perch. No way for the gramma, clowns or other gobies. (unless you research and find SMALL gobies)
I have a pink spotted goby that started at maybe an inch long and he is every bit of 5 inches now. Hectors gobies stay small, hi fins stay small....you still have some options.