Is the Skyline legal in the US?

Generally, no, but it depends, its all about your state specific laws. Its generally because of them being grey market cars, which means that if they find out that you had one imported, or brought in from any country, it will be impounded and auctioned with no refund, but u can get around that in california, if it is registered out of state, and you can bring it in that way, or there are a few companies that make the body of the car in japan and bring them up to DOT/EPA standards, and ship them as parts, which then, the EPA doesn't pay to much attention to them anyway