Okay, thanks for the advise, i'll try to keep it in mind when working on new pixel art pieces.

Yes. There's no requirement that says pixel art has to use as few colours as possible, but there are benefits to it. Most importantly, it helps avoid noise and keeps things unified in their colour scehemes. It also makes the colours easier to edit later if you need to, since you need to worry about fewer colour relationships.

Pixel art is clearest to read when it relies on clusters of colour to convey information. The more colours you have, the smaller and more numerous your clusters are, and the more they risk looking like noise (unwanted detail, information that's not what you meant to convey). For example, the light shadows around the buttons read more like an unintentional blur than a shadow, and the lightest colour on the tongue reads like a highlight that gives the tongue a weird shape, rather than as part of a gradient (fortunately this last one is only noticeable zoomed in; at 1x the entire mouth may as well be two solid colours, the contrast is so low).

In general, I recommend starting with as few colours as possible to get your idea down. Pick the opposite ends of each ramp (e.g. the darkest and lightest grey, the lightest and darkest green, etc), and draw everything with just those. Then, add more colours to smooth things out if you need to. Whenever you do decide to add a new colour, look at your existing colours first and see if one of those will do the job. This might also help you avoid the recurring problem you have with adding details that are invisible without zooming in because of their low contrast.

Lastly, spending your time on picking and using colours that aren't even distinguishable to add nearly-invisible details is just not a smart use of your time :] You could use that time to make more art instead.

I'm still fairly new to making pixel art, is using a minimum amount of colors typical for pixel art? 

You have 43 colours in this piece, a lot of which are nearly-identical greys. A lot of the details you added are only visible when you zoom in a lot, and are invisible zoomed out, in particular the faint shadow around the buttons and some of the gradient in the mouth. Here's an edit preserving almost all of the details (even those faint shadows), but using 26 colours, meaning I've cut 17 colours. Check it out, it might help you find areas where you can reuse colours in the future.