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As far as I know they're legal to keep in the whole USA, although some species have restrictions on collecting, and blue-rings are toxic, so there are restrictions (but not as many as there should be) on shipping them.
See the articles in ceph care for details on tank requirements, but in general, cephs need a large water volume, so we recommend 55gal for the octos that are best as pets (bimacs and briareus), and at least 30gal for the smallest cuttles (bandensis). There are dwarf octo species that can handle a smaller tank, but they tend to be nocturnal and short-lived, and all cephs are very sensitive to water quality, which is harder to keep stable in a small tank.