Strictily speaking you shouldn't have an 18g. piercing on anything that may become traumatized. It is inappropriate (in more opinions than just my own) to have even your ears pierced at an 18g. The reason for this is that the guage of metal is simply too small to disperse any stress that may be put on the piercing which could lead to a tear out. I would highly discourage this gauge of piercing except for things like lingual frenulum piercings and eyelid piercings.
18 Gauge
18 or 20 gauge
It is normal for a lobe piercing to be 16 or 18 gauge... but it is normally 18. A 16 gauge is normally used for tragus piercings :)
Use a taper or have a shop taper it out.
For a standard nostril piercing, 18 g.
18-16 Gauge
You should use a taper. You can probably find these at a piercing studio. It is important to first stretch it to a 16 gauge, and then to a 14 gauge about a month later to prevent tearing.
Barbells are used where a piercing is desired using a straight piece of jewellery. The gauge "18" is the AWG size of the jewellery. (American Wire Gauge)
Normal ear piercings are 18 gauge.
It depends on where your piercing is, but usually you will get between a 14-18 gauge. (Smaller gauge = bigger hole)
For piercing, 18 gauge. that's the same gauge as a standard earring. There is also a 20 gauge which is a little smaller than an 18 gauge and rarely used for body piercing. There are smaller guage needles for medical use. 25 gauge is the smallest typically used in outpatient health care settings.
Most "normal" piercings are 18 gauge, so yes, I dont see why not.