Help a wannabe writer of you can.

Sure; your title has a typo.

Well, one thing is that the kind of jewellery used in nipples isn't going to be the same as in ears. You can kinda use actual hoop earrings as a ring in other places, but it won't be a great idea. Most of the time, hoop earrings have a very thin part that goes through the ear, as in thin enough to essentially be a wire.

The way nipples get pierced leaves a bigger channel through the nipple. So, you use the thin wire in that hole, and you get irritation as well as a higher risk of it causing trouble when it gets snagged because a thinner gauge is going to pull through skin easier (up to a point). Most hoops for ears also use a different way of closing. The little wire gets put into a hollow in the opposite end, whereas body jewelry tends to be secured by balls that are either screwed on, or are pushed into place and secured by divots in the ball.

I'm not saying that nobody ever uses regular hoops in their nipples, I know plenty of people that do. I'm saying that you wouldn't mistake one kind of ring for the other; there's really not much visual overlap.

As far as how you would write the kind of idea you're wanting to express, that's a more complicated question. I tend to favor such things coming out via dialogue. Two old friends talking about the things they've left behind, there's a dozen ways to say that one of them has stopped wearing their body jewelry.

I think it's a good way to show-don't-tell. You could add a few more of these interactions to develop the plot point and make sure readers don't miss it the first time around (e.g. if the reader doesn't know what a nipple piercing is, or doesn't associate it with teen rebellion).

But does/can my idea hold any water? Is it credible? Or am I trying to put too much weight on a twig?

There is no such thing as 'too much weight on a twig'. At least, in theory. And it's even worse as far as 'credibility' goes. It's fiction and it's prose. Anything is 100% credible if it works. Anything becomes a(n abject) failure the moment it doesn't work.

I mean, it depends on your story (the characters, but also its scope and pacing, the way you focus or chose not to focus your reader's attention on other details, and so on), your style and mastery. It also heavily depends the type of reader you (want to) write for.

Sometimes neat ideas will work wonders, more often than not they won't work that well. You need to write it down to see if it works, or not. That's one of the reasons why, imvho, the process of working on a story should be much more accurately described as editing a story or even better re-writing that damn thing for the umpteenth time! ;)

Character details should really only be expounded upon if they drive the narrative forward. You don't just sprinkle them in for the fuck of it if they serve no purpose. This can be in a small way, eg a prominent scar that drives the immediate conversation towards some other engagement, its relevance lasting half a page. Or in a more long-term way like an adverse reaction to an older male stranger nearby, hinting at a violent childhood experience that manifests as an avoidant behaviour, which may drive further recurring narrative elements, or be a challenge that must be overcome as part of the character arc.

If the nipple piercings never come up again in the story, but the character still shares about them unprompted, this could still work towards character development by being a hint that the character has Aspergers.

One of such details I want to insert is one of the protagonists having had nipple piercings, which are no longer used. This is not openly stated; I just go into a very detailed description of a piece of jewelry that can be mistaken for coventional earrings and have a dialogue between the characters where nothing is really confirmed nor disproved.

I feel like the part of the character you want to portray by using nipple rings won't come across if the reader doesn't know that they're nipple rings. What is the purpose of describing an object if we still don't know what it is, and then miss the implied background of the rebellious years? Do we care to solve the mystery of this "jewellery = nipple rings", or do we care that "nipple rings = possible troubled past"? You can call it what it is, and have the option to elaborate more later if it serves the story.