Does NIGHTS know Sonic the Hedgehog?

Answer 1

Possibly. Although they have appeared together in various titles, they don't appear together in a main series game-only in spin-offs. The only exception to this is in Sonic Adventure, where, in the NiGHTS pinball table, the character appears. But he has no reaction that differs from the on that Tails gets when playing the level. Even then, it could just be a model, built into the casino. In the spin-offs, they will probably know each other.