As I said in my previous post ISIS never pretends to be someone else. They want the world to know that they are the terrorists. So that allegation is just baloney.
Unless you were there to witness what actually happened all you have is speculation. That's all any of us can do at this point.
They may have watched some movies where the bad guys dress up as cops or soldiers so they can get in to an area they otherwise would had a hard time getting in to. You don't know what they are willing to do for their false god allah.
For all we know they got a group of guys that shaved all their beards off, got a nice hair cut so they would look the part when they put on whatever uniforms they were wearing. That would be a very highly effective way to kill a lot of folks.
I'm surprised this has not been done in the US yet at a sporting event or concert. I guess we will see something like this eventually as this attack was very effective.
Besides, I saw an article where the US government was warning the Russians of a security concern 2 or 3 days before this happens so it may have been American CIA agents doing this for all we know to escalate things with Russia.
It's not like the CIA has not done stuff like this before.