Where to start. Okay, the shading under the hood? Good. It gives the characters a sense of mystery and menace. As traditional with chibi-work, the proportions portray a larger head than usual. In some works, this can go wrong, but you seem to have made it work very well. The lighting provides good focus on Ezio, as compared to the others.
The Doctor's nose could be a little longer; it depends on preference really. If you wanted, you could add a little more detail to the characters; fine tune them per say. If you wanted to send out a more menacing tone, maybe add some illumination to the characters' eyes? Not necessarily all of them, maybe just Ezio.
All in all, I really like this. It's different from the mainstream of fanart, but it still gives a good tribute to the game. You've done well on this. I personally like the cooler tone with the blues and the whites.
Ah yes, the brotherhood. A powerful network of multiplayable characters. So let's start. The overall tone of the piece still makes it feel dark and somewhat serious. I honestly would prefer if Ezio's eyes were not seen at all. By blocking it out, it gives a better "assassin" feel. I think the doctor and the priest were done very well. There is a good balance between being overly adorable yet not silly that usually comes with the idea of chibi art. Your detail of Ezio's armor is very accurate and well executed. I very much feel that this is a pretty good chibi rendition. This is probably my preference but I would've placed the characters a little closer together. Or, maybe have a gradual fading of color to accentuate Ezio more. My last suggestion is to give them a more signature feel, like the way the doctor has the long nosed mask and the executioner has the hood. The blues are divine and you captured the Animus feel very well.
Overall, I say well done. Chibis done not for over the top comedy but as an alternative perspective is well executed. You have accentuated my live for the AC brand even more. I want to our and find plushies of these to snuggle with or display proudly. Insieme per la vittoria