It's a 20 long. I've had bettas with shrimp before and no problems and many people have had success as well. I've read up on rummynoses and I've found that a 20 long is suitable for them? I do really like cardinals though, and since they're smaller and less bioload, I'll probably do them instead. Unfortunately, I only have lfs who sell splendens, no imbellises. I've also read that keeping a male with female bettas is a no-go? I don't think that's a smart idea, and I definitely wouldn't feel comfortable doing that. But I am confident in my decision to do a betta sorority. Horror stories happen when they haven't done enough researchIts a 20 high or long? Also what type of filtration?
The bettas will eat the shrimp, maybe not all the bettas-but if half of them eat the shrimp you will have no shrimp.
Go with a clown pleco if you need one in a 20, a BN pleco will produce too much waste. For a 20 with your other stock, I would go no plecos.
TBH there is not enough room for rummys; the rummys are very active and need the space to move, minimum of 36" is preferable. You can go with 10-12 ember tetra, or 6-8 cardinal or glowlight tetras. You can even stay with the Asian theme and use 10-12 hengli or espei rasbora, they are pretty and also active but better suited for a 20 long.
If I was going to have a 20 gallon and wanted bettas I would look for wild bettas that are social and can safely be housed together like imbellis. I have read too many horror stories with multiple betta splendens tank where it did not end well for many of the tank's inhabitants. When male and female bettas are introduced for breeding, it is usually the female that will fight and kill the male if he is not acceptable. I would not include shrimp as bettas eat invertebrates. I would just set up the tank for the bettas; a male and 2-3 females, with lots of leaf litter and some rocks and driftwood, maybe add some kuhli loaches like pangio or oblonga.