A comic I commissioned TFSubmissions for! (They drew it, colored it... Everything! All the art is by them! Not me! Need to make that as clear as possible! All I did was design the characters and write the script)
https://www.deviantart.com/tfsubmissions

So over time the rules were, sort of, established. As best as I can recall off the top of my head, they are:

1) There are tiny imps, and normal human-sized imps. Tiny imps are around 1 ft tall. They can switch forms, but it is either out of their control or at the very least temporary!

2) They have yellow eyes. The iris can be any color, however.

3) The shape of the ears is flexible, but they must be fluffy and big

4) Fur color is also quite flexible, usually leaning towards darker colors with a light area around the mouth/body

5) Tail can vary quite a bit as well. Usually they are mammalian in design but there have been imps with bird-feather tails!

6) Wings are an optional trait.

7) Body type is pretty much "anything goes". The only constant is the chibi style proportions, with larger heads, eyes, etc.

8) Imps are cute! So they are not vicious nor do they have terrifying magical powers. We do discover that some have some sort of power, but it is rather limited and benign. This also goes hand in hand with tiny imps being... Tiny. They aren't super strong or anything, they're shrunk people!

9) Seems that imps are all female, so males are turned into cute impy girls! This isn't 100% though, I can imagine there being a male imp somewhere

10) Rarely ever comes up, but I like to always imagine in my head that tiny imps have cute, squeaky voices due to their vocal chords being shrunk down :3