Flybum, if you want to keep it low tech, I would get some plants like Anubias, Java ferns, crypts, maybe some moss. These are plants that grow pretty slow, and don't need a lot of light or maintenance. You will need some kind of root fertilizer (tabs or dirt do the trick) for the crypts though. A slightly heavily stocked tank will should be enough fertilizer for the others. You may want to look into Otocinclus catfish (otos) instead of cories for that tank though, 3 cories might get to be a little much for the footprint of that tank once they are fully grown.
As for tannins, charcoal filtration and frequent water changes should keep the water pretty clear until the tannins finish leaching.
Also, fishless cycles are fine, but if you plant a tank heavily upfront, use seeded filter media that has beneficial bacteria in it, (you can pop a sponge or other media into the 5g tank for a week or more to get beneficial bacteria building up on it when you are getting ready to set up the 55g....or even just steal some filter media straight out of the 5g, call it time for a media change
) and keep an eye on the ammonia and nitrIte levels and do water changes when and if needed, you can do a cycle with fish. You just don't want to start out with the maximum number of fish. Fast growing stem plants (even ones like anachris which is cheap because mostly it's used for turtle food) and floating plants are great at soaking up excess ammonia and nitrIte (nutrients to the plants). I have even tossed stems in as floaters to do that job, and then removed them later. The added benefit if you get the plants locally is that they should still have beneficial bacteria on them.
OK that was a lot of information. If you are not familiar with what a fish tank cycle is, THAT is really important to know. But if you do it right, fishless cycling is not strictly necessary (I know kids (and adults too) can get impatient to get fish in the tank).
And I do recommend reading as much as you can, but it can also get confusing, so don't hesitate to post your questions as well