Basically, the possibilities for this type of set-up are endless. The nitty gritty detail all lies in how strict you want to be with the biotope and if you want to model a particular "area".
A lot of people use either a light coloured sand or a very dark (practically black) sand for the substrate. I've read boiling oak leaves makes them sink, and so you can have areas of leaf litter, however to prevent decomposition these would have to be replaced fairly regularly.
You are fairly restricted on plants unfortunately, as not a huge amount of amazonian plants are commonly sold. Amazon Swords are, as you know, the most obvious, and the variety of forms available mean you can create quite an aquascape (however it would be rare to find two types in the same place in the wild and therefore this would not be biotopical). On top of this there are brazillian milfoil, brazillian pennywort, amazon frogbit, various stem plants such as vallisneria etc etc. Of course, there are vast areas where no underwater plants are found, chiefly in areas of flooded forest in which you'd typically find discus.
Now for fish. You could get away with a pair of angels in a 30, however I wouldn't have any more as this isn't much space for them to "breathe" if they become territorial. An alternative centrepiece fish would be flag cichlids, again a pair would work beautifully in there, and then maybe a pair of apistogrammas (depending on where they're from). As for tetras, lots actually don't come from the amazon, and so you'd want to research and find tetras which are from the amazon, such as cardinals and pristella tetras, hatchet fish etc, but you're certainly not limited here. Corydoras catfish again span a huge area, from southernmost south america to the bahamas (where species such as bronze corydoras are found). Depending on how strict you want to be you could research on corydoras and other catfish that typically derive from the amazon.
My personal aim is to create a very strict biotope with wild-type discus, flag cichlids, and pristella tetras to mimic one of the many flooded lakes left during the dry season.