shrimps and carbs

My daughter has a small tank with just a Betta and a few neons in it. Recently she bought 2 crabs and 3 shrimp like things. the fish shop said the shrimps came from Thailand. They are a sort of blue/grey transparent colour, about an inch long. I told her that neither would be likely to survive but surprisingly, 3 weeks on, she's only lost one shrimp. It's aroused my curiosity now - they are quite fascinating to watch and seem to cohabit quite comfortably with the fish. Are there any sites that can give any info on the conditions in which they thrive and what things to be careful of? Also, re the crabs, I always thought crabs needed somewhere to climb out on to but these don't seem to. However, I can't find out anything about optimum conditions for these either.

Or even 'shrimps and crabs'

I thought this was a low carb group.

RE crabs: It depends on the species whether they need to get out. Fiddler crabs most certainly do.

Its usually not a good idea to keep shrimps & crabs together. Shrimps can carry a disease that is contagious to other crustaceans. If the crab was to eat them it could get sick & die. Also not all crabs need places to get out of the water. Check out this site.