kveeti is right that a few more black skirts would be a good addition to your new tank, as the fish do like to school. They are also peaceful and hardy, and would go well with a number of other tetras, such as neons (which you yourself suggested) and head-and-tail lights. If you don't mind breaking the South American "theme", harlequin rasboras would be another interesting addition. With a tank as large as a 55, you could get 7-9 each of three groups, and not be stressing your tank out at all.
With all of the fish listed above, corydoras would do well as bottom fish. Otocinclus would be good for algae eating, but they suffer during shipment to fish stores and there is fairly high mortality after they get into hobbyists' tanks, so for that job you might want to consider one of the fairly small peckoltias (another type of algae eater that looks something like a pleco) or a chaetostoma species (sometimes sold as the "rubber" or "albinus" pleco).
Floating plants and a fairly dark substrate will make all of these fish feel more comfortable and enhance their colors. Also, your Emperor 280 will need a little help, although you don't have to buy that at the LFS. There are on-line retailers such as Big Al's or Drs. Foster & Smith that sell those units for 50% less than any LFS (I would personally suggest that you supplement your Emperor with an Aquaclear 300 loaded with nothing but sponges, which will give you additional biological filtration, better mechanical filtration, and you won't have to buy filter cartridges for. You can get one of those for $23.00 on-line).
Also, if you really only have 4 black skirt tetras right now, you don't need to worry about the quarantine thing, as there isn't really an existing tank population that you are putting at risk. However, expecially when buying the neons, look carefully for any lighter/whitish colored areas on any of the fish in the tank, as that would be a sign of neon tetra disease, for which there is no cure.
Good luck!
