They can be similar in size, some will be bigger and some smaller. And they will not breed or attack each other. I have two cherry shrimp, one ghost shrimp, and one other type of shrimp. I have some in with Caesar. I put Java moss in there and they seem happy enough. But if you put them in with a Betta, depending on the Betta, it might or might not eat them. Caesar only attacked Ghost shrimp, while they were originally in with King, who would eat any of them.
Bettas: Rose, Sophia, Addie, Ghost, Buster, Holt, Aidan, and Mercutio!
Just my animals: 0.2 Cornsnakes, 0.2 Bearded Dragons, 1.0 Water Dragon, 2.2 Crested Geckos, 1.0 Tiger Salamander, 0.0.1 Tadpole, 0.0.2 Red Eared Slider Turtles, 0.0.1 Map Turtle, 0.0.2 Hermit Crabs, 1.0 Hamster, 1.0 Rabbit, 0.1 Horses, 2.2 Dogs, 0.2 Cats, Chickens, Ducks, Oscar Tank, Planted Bowl, 10 Gallon, 4x 0.5 Gallon, 5 Gallon, and Planted 25 Gallon Tank....Is that it?