A group that size would be fine in a 20 gallon.
I would go tall so that the plants have some room to grow and you can do a few inches of substrate without sacrificing all their space.
If you choose to use an aquarium with a screen lid, a small computer fan would not be a bad idea.
Position it to blow across the top of the screen, not into the enclosure, to draw air up and out.
Have you read Tom Routhouska's articles on the e-zine?
If not, I highly recommend you do. He helped me a lot with my brevicaudatus, as did Mike Monge.
-Brad