What do you mean by "clean the sand"? Cories are great at nomming up leftover food, but won't touch poop. Some ghost shrimp do, others don't, but either way they poop it right back out the other end. MTS are wonderful at stirring up the sand to prevent gas pockets forming, but you can do that yourself with a chopstick or your fingers, with no risk of a snail infestation. Shrimp aren't that hard to maintain - in a 10g with just a betta, or a betta and some tetras, they will be fine with just a 25% water change per week (assuming you have a filter).
Definitely don't go with a crayfish, because it will be bye-bye betta fins if you do.
I really like the idea of an Amazon theme, with a betta, 6 neons and 6 pygmy cories. :) Cories and neons both being South American fish, it would look pretty authentic (apart from the betta). Neons look amazing against black sand with live plants. It really makes their colours pop. :) some good plants would be:
- java fern
- rotala wallachi (if you can get it in red it looks amazing)
They are all really easy to maintain. :)
I have sand in my 16 gallon, and it's easy to clean but I have to do it everyday to get the shrimp poo off it, and I have a Thai-themed 5 gallon. :)