As a rule, tetras and rasboras are a social fish and should be in groups of 6 or more. They will school if threatened by a predator or engaged in mass feeding, otherwise if not threatened, these fish break-off from the school and form little groups of 2-3 to look for food. Keeping 6 or more is a way to keep agression down. They are a social fish, and their "group" is comprised of a dominant fish, usually a male and the subordinates. Having six or more spreads the dominant's agression towards the subordinates. If you remove the dominant fish, the subordinates will squabble, sometimes violently for a "dominant" position. Less than the ideal number focuses the dominant's agression on too few fish, resulting in added stress and injuries. Corydoras should also be in larger groups, they actually enjoy foraging together. You've already been told that common plecos get too big, and you should rethink the clown loaches. An option for a 55g that's tall may be angelfish, which may appreciate a taller tank.
As others have said, I really recommend the amount of schooling fish. It is better to have three substantial schools of fish rather than a smattering of many species.
Sand bottoms are lovely but if you intend to keep plants, make sure that there is a nutrient-rich layer or a plant substrate of some type below the sand. Such substances included Laterite, Fluorite, Eco-complete, or ADA aqua soil. Prices range from affordable to pretty pricy, but there is something for every budget. In addition, you must make sure that you have burrowing snails, and disturb the substrate regularly to prevent pockets of anaerobic bacteria or what not forming when sand compacts. I forget the name, but it happens when sand compacts and releases hydrogen sulfide, not good, very bad for fish. Make sure you have enough light would be the only other thing I'd have to say for now. So many planted tanks go bust because people don't add enough light from the start. Take into consideration that your tank is tall.
Good luck to you, can't wait to see pictures when this one is setup. It's an unusual shape, so I'm interested to see how it looks planted. You can experiment with some taller plants, which is always nice, and possibly create a lot of depth to your scape.
llj
