Wait, wait. I need to figure this out, please bear with me.
First subject: If you want to have shrimp, the answer from Shellyx included the info that shrimp love a spray bar. Then I sent pictures of spray bars to answer your question of what they are.
Second subject: You're right, a betta won't appreciate the added motion from a spray bar. Or a bubbler, for that matter. They like low flow, little motion. Didn't know you have a betta in there, I thought the subject was only about shrimp.
Third subject: Plakat female bettas. I've heard from friends who have them that yes, they can be more aggressive than the long-finned betta splendens, as they were originally bred to be fighters (like the male betta splendens were).
So, do I have this right?--you have a 20g with a (male long-fin or female plakat, which?) betta, and a bubbler, and are wondering if adding the shrimp would cause fighting and the shrimp to go hungry because of bubbler action?
If that's the case, I can report from my experience that bettas, even aggressive ones, can sometimes give attitude to shrimp but there's no fight-to-the-death scenes going on. By that I mean they'll swim up, maybe even flare at them, maybe even turn around and give 'em the fin bump, then leave. Others may have different experiences.
Having just a low flow filter with no bubbler will certainly keep your betta happy. It won't steal shrimp food either, especially if you feed the sinking pellet variety. They love to nip into that as it dissolves on the bottom.
Hope this helps.