Of course you can. There are no restrictions to what you want to display your avatar as, as long as it is not offensive, meaning, as long as it does not display things such as nudity or violence.
People do it all the time. A lot of people of different races use human avatars. You are not disallowed to do what you planned on doing.
The only such rule that is posted is that you can't "fake game features," meaning you couldn't use one of the Combine's "under arrest" avatars to pretend you are under arrest and prevent other players from trying to arrest you. Otherwise you can pretty much use anything you want, within reason. Many players don't use pictures of their character at all as their avatar, and use some symbol they like, or something else entirely. It's all good.