Theoretically it could work with the betta, assuming he was fairly chill.... You'd lose quite a few baby shrimp, but as long as you weren't wanting to breed them, and made sure they had plenty of hiding places, it should be fine. I've kept shrimp with bettas before and had it be a great pairing, I have a betta right now that will murder anything, shrimps, small fish, he'll go after them even when they hide. It depends on the temperament of the fish. I'd say let the plants grow in pretty well before adding the betta, that way there's enough nooks and crannies everyone has their own space