I'll make this real quick.
These aliens unfortunately belong to Fox. Their shape shifting ability is their main power. That and their Super-Skrull program. This however is a hurdle that Marvel Studios can clear fairly easily.
The Chitauri are already introduced in the first Avengers movie. I know you are wondering how these guys are comparable to the Skrull. Watch below.
Okay never mind then. I can't find a video on YouTube so I made my own and put it on Facebook but it is apparently not embedable. Watch Ultimate Avengers to see what I'm referring to.
Back on track, the Chituari have shape shifters. So Chitauri shape shifters can be brought in.