Butterfly needles
Yes, the larger the gauge the smaller the needle size.
Yes. You get it pierced with a 14g needle instead of an 18g
Diameter
25 gauge is the size of the needle not the syringe. they are measured in cc's
No - the two have no relationship to each other at all.
21 gauge
Gauge is a measure of how large the opening of the needle is. The larger the number the smaller the hole. Most venipuncture is done with a 16-24 gauge needle, depending on the size of the patient and the location of the vein. A 10 gauge needle would be huge.
It is determined by the size of the veins
Size and condition of the vein
Guage 18 in medicine is the size of a needle either for injections or for IV insertion.
The smallest gauge needle that I'm aware of is 22g. But no one will pierce you with anything less than 20g, and that's usually in the nose. But, the standard size piercing needle is around 16-12g, but you can make a request to be pierced at a larger gauge, if there's sufficient flesh for it to go through.
It depends, sewing needles come in all different sizes.