In most cases, the X File comics are canon. They often expand on the storylines and characters established in the main series, providing additional depth and context.
In general, the main X-Men comic series is regarded as canon. However, there are special crossover events or spin-offs that might have their own unique rules and continuity. It really depends on the specific context and the editorial decisions made by the publishers.
It depends on your perspective. Some fans consider X-Men First Class canon as it ties in with certain comic arcs and themes. However, others might argue that it deviates enough to not be fully canonical.