impact barrel removal

Yes, it is just fine. Fully close the cocking lever first.

I would just never fire one to totally depressurize or if the air bottle has been removed, but as long as there is air in the bottle, dry-firing is fine.

In fact (in the future, perhaps), to adjust the regulator pressure down, you need to dry fire it as specified in the FX Owner's Manual. You make small adjustments to the reg, dry fire 2-3 times until the reg settles at its new pressure, and repeat the process until you achieve the reg pressure you want.