Yes a humidifier is fine, just make sure it's not blowing a cloud of vapor directly at your light.
Inline fans are important, they increase air quality by fully cycling all the air in your tent in under a minute. They also help regulate your humidity which will be an issue a few weeks into flower.
Set you fan up to be blowing out, and open a vent or two on your tent. When your tent is closed, the walls should suck in a bit (even with a couple vents open). This means your circulation is good.