I'm not sure how small you want to go but I'm really enjoying the idea of having my 5g shrimp tank. No fish, just some shrimp, some marimo balls, and maybe some flame or star moss if I can find it. What's even better is that it's on my kitchen counter right next to my sink so water changes will be a snap!
Shrimp in a 5gal is good. I like a Betta in a 5gal to. What size tank is 'smaller' though? There are alot of options that are smaller than your current 50gal. Having an idea of what size tank you are thinking of would help alot for giving ideas.
It's a common belief that smaller is easier to maintain, IMO this is not entirely true. All setups require regular maintanence (e.g. weekly-fortnightly water changes), but with a smaller setup, the volume of the change is much less.
One thing I've noticed with going larger (I went from 5G to 16G) is that my parameters are a lot more stable. A larger volume of water takes longer to alter, temperature and other parameters take longer to adjust.
That said, I'm not trying to discourage you from smaller setups, just wanted to dispell a little myth (IME).
Small setups are great, you just have to be a little more on top of things in the early days to get the right balance.
How about a couple of African Dwarf Frogs in a 5 or 10g tank? They're cute and can be comical.
I have 2 adults in a 5g mini bow, but I think they'd be happier with a larger foot print like a 10g.
The 6g bookshelf tank would be good for them. It's got a long foot print like a 10g but not as tall.
i think the small tanks are pretty cool....they can be a little more delicate to balance....but i think once you do, it's fine. the amount of water you change in them is small, so a bucket would not become heavy, and is manageable. on my 20g, when i change even 5g of it, i have a very hard time carrying it. that is kind of a "chore". but i'm a whimpy girl haha.
i have a 5 gal hex with a female betta, and a 10g divided with male bettas. i don't know if you wanted to consider bettas or not. i did have one in a 2.5g, but he's moved to the 10. all that's left in the 2.5 are two nerite snails. i'm thinking of making it a red cherry shrimp tank.
bigger tanks aren't mistake proof, but are a bit easier to balance, and control temp. on a small tank the lights heat it up faster....and i use cfls. but if you have an led or something, that would be ideal.