I'm trying to hook up a light fixture in a garage. The existing fixture is connected with 16 gauge and I'm trying to run 14 to another fixture. Is that safe?

Yes, the conductors being used by the fixture are designed for the current draw of the light.

Before I say yes, I need to know exactly where is the #16 coming from.

I agree. You should not have ANY #16 house wiring. I don't even think it exists.

Well am I guilty of misreading the OP?

The statement, "existing fixture is connected with 16 gauge and I'm trying to run 14 to another fixture."

Clearly positioned the #16 as part of the fixture and the #14 going to another fixture.

Did I miss something?

I am not sure. The OP is very vague.

Is the fixture #16? Or something else?

If the building wire is #16, that is wrong.

If the building wiring to the next fixture is connected to #16 in the fixture, which is part of that fixture, that is wrong.

The #14 wire from the new fixture must connect to a minimum of #14 feeding the existing fixture.

If the building was inspected, then if they used #16 shouldn't that have been discovered at the rough in inspection?

Unless, of course, the poster built the building by himself and used the wrong sized cable.