I'm going to start publishing my design notes for the different sapient species, but rather than have the appearance of each creature be definite, instead I want to keep a variety of imaginative descriptions that allow each player or games runner to effectively create the species as they see fit.
This is because I don't want to limit other's creative visions and instead tie them to how I think everyone should see each creature. Not least because I also think this helps develop the idea that magick has permeated through all things, such that each evolutionary channel for a species exists simultaneously.
So each species can have as many of the descriptive 'extras' that I will use, but they will always have certain core components that should probably be kept as this explains their species psychology. Note that I'm suggesting that appearance can and should be customised but that I feel the 'role' that each species psychology plays in the elucidation of the world view should be left as I have attempted to describe it. This is because, for me, each species is an aspect of abnormal psychology that greater describes the 'puzzle' of the story world, in that it is a mirror of human creativity, frustration and loss.
Let the player choose their own interpretation, some will want it to be defined for them and others will want to set their description wider. Give all approaches a chance. For the games runner their choices can affect the entire species or merely change individuals, as mentioned, both can be allowed. Ultimately what goes is left in the hands of the story teller and should be used to better develop their story/word.