If your tank has cycled (ammonia and nitrite have risen and fallen to 0), then you can add a shrimp at any time. Depending on what you want to keep, some shrimp are better than others ie; reef safe. Horseshoe crabs, on the other hand, are not good aquarium inhabitants. They get way
too large for a 20g, I have seen them as large as 24" in diameter here in the gulf. You won't have a prob there though, cause they will starve long before then. They eat critters out of the sand bed, which I see you don't have a sand bed, cause your using a UGF
. The crab would be pretty uncomfortable not being able to get under the sand.
I'd also suggest that you lose the UGF
, they are difficult to maintain and will produce copious amounts of nitrates.