Hey, thumbs up for this idea.
If you look on the left here under the tap "Aquariums" under my name you can see my various "Shrimp farms"
You can set up a 5g; but my pers pref over the yrs became 10g's for shrimp farms.
You best set up the tank with various small leaf live plants and Java Moss
, they LOVE Java Moss
you can see mine acting like cows on Java Moss
I have one of my shrimp tanks currently stocked with a Betta, no problem. It really dep on what fish you want to add there, some will go after the shrimp others won't. If you want to seriously consider breeding them, I'd not add fish.
Also the best you can do for your shrimp is a sponge filter, any regular HOB or canister or internal filter will suck them up and you don't want that (sponge filter cost like ~$6-7)