You can probably keep a bunch in an 8g, but the more you have in there, the more often I would clean the bottom.. which I'd leave bare. A sponge filter would be good as a regular
HOB will suck up the tiny baby shrimp.
The cleaning challenge is the baby shrimp that you suck up. I spend more time moving the removed tank water from 1 tub to another checking each cup of water for baby shrimp than I do cleaning.
A ton of java moss or guppy grass would be good along with some caves and even some filter sponges (the baby shrimp love hanging out on them).