Is it possible to go from 16 to 12? In other words skip a size?
From a 16 to a 12 is nothing at all.
I began my ears with a 10, and worked from a 10 to 6, that hurt a little, but it only stung for about 15 minutes.
I'm currently at a 4, and I'm probably gonna stop at a 2 or a 0.
Your question about skipping sizes: yes, you can.
I suggest probably even going from 16 to 10, because 10 is a good starter.
12-10.
The best way to do this is by going to your local piercing/tattoo parlor, and having them do it...otherwise you could fuck up and bleed a lot, which would suck, and you'd ruin your ears.
It usually costs from $20-$50 for starting, and $5 per revisit for stretching, so it's not extremely expensive, and you HAVE to take good care of them, or else you could run the risk of infection.