1. Should changeling imply fim? On the one hand there are pics that contains only OC changeling, on the other hand this species is unique to fim.

2. Should changeling imply any other species tags? Many people are tagging them with equine or even pony.

Changeling unique to FIM? The fuck are you smoking? That's been around since ancient Greece. If you want a tag to represent FIM changelings, try changeling_(mlp) or something.

Check the usage of changeling tag. I've failed to see any changeling in other meaning beside FIM. But if that really bugs you that much, then consider that there would be done alias changeling -> changeling_(mlp), and we will wait for changeling_(greek_myth) tag to appear (highly unlikely). Those 2 questions are still valid.