On the back "wall of swords" idea, I would not. Creating a solid wall of identical plants immediately draws attention to the "wall." One trick in aquascaping is to always have at least one spot where you can actually see the rear wall [a plain black background makes this spot look deep, off in the distance]. Also, a row of the same plant looks artificial when it is something as distinctive as swords. Vallisneria is fine together because it naturally grows in groupings. But with the larger swords, spacing 3 or more of them intermittently works better, or in very large tanks a group of 3 can work.
On the sword leaves. Echinodorus are amphibious bog plants, not "true" aquatics. They spend half the year emersed and half submersed due to the flood periods in South America. The emersed leaves are different from the submersed because they perform a different function and emersed leaves need to be stronger to stand up without the support of water. When we buy these plants, they are usually in the emersed state, as nurseries find it less expensive to propagate them emersed. Once planted submersed, within a few weeks the existing leaves, if emersed form, will yellow and new growth (from the centre of the crown) will be the submersed leaf form. We have several common species of Echinodorus in our plant profiles, and one of them has a photo of the plant with both leaf forms; can't remember which species it is, but there are not many and you can check them. That will give you an illustration of how different the leaves can be on the same plant.
The link you posted to the "Amazon Sword" is Echinodorus amazonicus (as they name it). It is nearly identical to Echinodorus bleherae. Rataj described these two species as distinct, and generally it has been maintained that E. amazonicus is a bit smaller than E. bleherae. However, subsequent botanists have suggested that these two species do not even exist as distinct species, that both are actually another species, E. greisbachi. Lehtonen did cladistic and DNA studies only a couple years ago to prove this [I reference this in our profiles]. I have left the species in the profiles with the names we all know them by to avoid confusion, but we can expect this to change in due course. Rataj has 62 distinct species, Lehtonen says there are only 28. Anyway, you can see these plants in my tank photos too; E. bleherae (so-called
) is in my 115g, and E. amazonicus in my 70g. I also have some adventitious plants (daughter plants) from E. bleherae growing in the 90g and they are now twice the size of what appears in those photos.