With CPOs...Orange Dwarf Crayfish. They are so cute! A couple of mine are spotted and a couple are more striped. I have two in each five-gallon tank.
They are plant, snail and shrimp safe and are slower than Si and Willie. They also spend most of their time on the substrate. I have spent almost all day watching them. I'll keep you updated on how they do and if they have any downside. But Willie and Si have ignored them and they've ignored Willie and Si. They seem to be most interested in cleaning the substrate.
I have hard water and that doesn't bother them. Also got some Thai Micro Crabs but they are spending most of their time hiding. I think they aren't very brave.
They are also darling little critters with their sideways walk. Neither adds much bioload.