Acestrorynchus falcatus and altus are probably the most common FW cudas available in the trade. They grow around 8-10" and need a lot of open space so a 55g is way too small. A. isalinae is a dwarf cuda that only grows to around 4".That would be the only real option for a 55 but they aren't easy to find. There's another species that I've kept that looked a lot like isalinae when young (prominent horizontal black stripes) but ended up losing them and growing pretty big, so watch out for that.
Best to keep them in a school (~6 would be good) without tankmates. They can be nervous and can get picked on by other fish. This can lead to them crashing into walls, or developing fin rot, which they are very susceptible to. If you do keep them with tankmates, make sure they are at least as large as the cudas and preferably not narrow bodied fish, otherwise they may end up as food. Best to avoid other shiny scaled fish as well. Some sort of catfish might work.
They're also jumpers, so make sure your tank is well covered - I lost 2 due to jumping through the gap in front of my HOB filter.