That's a silicone o-ring. I, personally, find that they help with adjusting fit on jewelry, so I try to include them on any longer posts and, eventually, with all of the gauged posts.

All the jewelry from MYM is uncoated. That means that the silver and brass aren't protected from patina. Some people are allergic to the coatings put on jewelry to protect it from patina. When it gets to you, it's just been shined by hand on my jewelers bench, so that's the best time to add a coating if you want one. Instead of a coating, I suggest using Sunshine Polishing Cloths to shine jewelry and keep it looking fresh and new forever.
If you don't want to shine, the easiest and cheapest coating to use is a clear nail polish to which you know you're not allergic. It will chip off and can be replaced as needed. Remove it with nail polish remover and redor. Note: Only do this with metal jewelry, not opals or stabilized gemstones!

Ack!! No!! Not ruined at all!

I don't use plated metals, so your jewelry is absolutely not ruined! All you need is to get a polishing cloth (I suggest a "sunshine cloth") and wipe your jewelry down. In fact, it will look EVEN BETTER than it ever did because the texture of the design will start pop! Well cared for metals are like fine wine. As long as you treat it nicely, it will look better and better as it ages. Just be sure that you run a sunshine cloth over it once in a while.

Protip: Save your sunshine cloth in a baggie and cut it into 1"x1" squares. Use each square until it's black and shiny. Just protect the cloth from debris and always store each square in its baggie, or else you risk scratching your jewelry!

Anytime that a jewelry starts to turn skin green, it's actually due to something that your skin secretions and the air are doing to the metal. That color (the same color that the statue of liberty is) is a chemical reaction with copper.
This usually happens with brass, because brass is an alloy of zinc and copper.
Whenever brass is heated to the temperatures needed to solder it to steel the more-reactive copper atoms are drawn to the surface of the metal. I remove the copper that does escape the alloy, but if you leave brass exposed to moisture and skin long enough, some of that copper that's still there will start to oxidize green.

Great news: It can be both prevented and removed fairly easily.

You can do this naturally or chemically.
The natural method is to just keep your jewelry clean and not leave it in when you shower. Prevent moisture from being trapped behind it, and you should be able to avoid that oxidization of copper.

You can also avoid metals with high copper content. It's found in brass, bronze, and (in much lower amounts) in sterling silver. (Sometimes on sterling silver there can be copper that ends up being around the solder point, although jewelers work hard to remove or avoid that. )

Chemical avoidance is probably easier. To prevent the green, you just need to protect the metal from moisture and air. Just coat it in clear nail polish, Renaissance wax, any other patina protector. No air = No green.

Without seeing you, I can't say. My automatic answer is to ask your piercer. Just give them a call and ask what they pierce with. You can even take your current jewelry into them and ask what gauge it is. They should be able to tell you.

If you can't do that but you know that you wear a larger gauge than a regular earring, then I suggest getting an 18 gauge. if you have scarring or the hole is in a groove, then I suggest an 8mm. Also, if your piercing was done in the past year, I would err on the side of caution and go longer. If you're still having swelling, the last thing you want is to have jewelry that's too tight.

If you don't, then try the 18 gauge 6mm.

If it doesn't fit, I accept returns.

This, like most things, takes practice.

Your piercer can help you if you absolutely can't do it yourself.

If you're doing it on your own, my personal trick is to take a regular earring and insert it through the hole from the front, and then meet it with the new post from the back, and push the earring out while inserting the new post. It straightens the hole and makes sure that you don't irritate the heck out of your piercing by stabbing aimlessly everywhere.

I have a tool that helps get the front in if you have arthritis, long nails, or stubby fingers. Look for it in the Add Ons section, and it's called the Handy Dandy Grabby Friend!

Due to the fact that this jewelry is made by humans, there's always a chance of something breaking. Because of that, I have a 100% replacement guarantee for broken jewelry for a 180 days (Etsy's limit). Just send me photo documentation of the break and I will replace it or refund you depending on the circumstances we discuss.

The Press-Fit or Threadless posts are the most durable ones. They can be tightened manually. They're my personal favorite.

After that, I like wearing split-rings with my EarKlips hooked on, especially those from the Lobestone line. Those are easy to switch out, but stay on really well the rest of the time. They also dangle and can attach to the split ring without having to open the ring itself.