A 20L should be fine for zebras. As others have said, it takes very little bioload to get things started so if you get that tank, please keep the same stocking level until the cycle is complete. The larger tank will change water quality more slowly and give you the chance to keep the water quality adequate for the fish you have. If you give in to the temptation to get more fish, you will create maintenance problems for yourself and your fish.
As far as rapid moving fish, the only thing I have seen that move as fast as zebras is endlers live bearers. They are a little smaller and very active swimmers. They should not be kept with guppies as they can crossbreed with them. If you are getting a second tank, they could be kept separately from the guppies and give you another active fish in a different color than the zebras in your tank. See my avatar for a blurry example. Couldn't get a good picture, he just wouldn't hold still.
As far as rapid moving fish, the only thing I have seen that move as fast as zebras is endlers live bearers. They are a little smaller and very active swimmers. They should not be kept with guppies as they can crossbreed with them. If you are getting a second tank, they could be kept separately from the guppies and give you another active fish in a different color than the zebras in your tank. See my avatar for a blurry example. Couldn't get a good picture, he just wouldn't hold still.
