not that I know of.. shrimp are the smallest, snails are small to big in size, then there are bottom dwellers but I wouldn't suggest them unless you have a 10 gallon tank or larger.
The betta may eat some. Ghost shrimp are really great for completing the ecosystem of a planted tank because they eat dieing plant matter then breed and/or die and give the betta a nice snack of dead shrimp and baby shrimp.
They do molt (shed their outer skins) so you need to provide lots of plant cover or a cave that the betta can't get into or stuck in where they can hide while they don't have shells
Got a dollar worth of ghost shrimps. They were supposed to give me 6 but when I got home there was only 5 in the bag :( :( :(
they're very tiny. once the ghost shrimps were placed in the tank the betta played hide n seek with them, trying to take a bite out of them but the shrimps were too fast for it. plus the shrimps are safe hiding inside the hornwort maze :)
I can't say. I've got three ghost shrimp in the tank with my betta and some live plants and other fish, and they've been doing just fine. I haven't lost a one of them. They're a great little cleaning crew though.