Bettas will occasionally eat shrimp too, it just depends on the individual fish.
I'm going to have to agree with Kristin on the rummies. Mine are constantly cruising around my 55 gallon discus tank, plus they can grow to nearly 2 inches TL- seems to me by the time you added an acceptable number for a school, it would be an awful lot of fish for only 6 gallons by the time they reach adult size. I like the idea for dwarf puffers, but again, you'd have to nix the shrimp. Honestly, I don't see why it was recommended to you not to keep either Boraras or galaxy rasboras. They probably aren't any more active than rummynose tetras, and at least their adult size is smaller. I think either species would look awesome with some cherry shrimp.
One last thing- have you thought about how small shrimp like cherry shrimp will fare, given the way the intake is set up for the filtration unit? I've gotten around this problem, of course, by making a sponge prefilter, but it seems to me that JBJ's filtration system (even though I think it's awesome) would pose a lot more of a problem in that regard.