They aren't special forces. Some may come from a special forces background but it wouldn't be a high number.

So what are they?

Ex military or crazy yahoo.

Surpisingly enough...having sat on a Trans Pacific flight next to one....they are often guys who can't get into the military for one reason or another.
And a number of them are also guys who were military and didn't fit back into civilian life.

But did they are US property ? or just running for contract to contract ?

The PMC companies based in the US are not Government or owned by the US. Some have had contracts from the US Government for different jobs overseas but they are not owned by the Government, it is still a private organization.

Just get a black vest which holds your mags and other stuff (I wouldn't really go with a plate carrier though) and underneath just wear a tshirt or maybe some brown or greyish coloured shirt/long sleeve (or whatever else you think is comfortable). For pants I would also get some black or khaki coloured pants, just something that doesn't have a camo pattern on it. for head gear wear goggles or safety glasses that look like sun glasses and wear a black/khaki/greyish baseball cap (but not those "gangsta" Bape or G Unit crap hats).
Thats what many PMC look like in pics.
Or you can dress like the guys from Army of two with their skull masks xD

Are polo's no longer worn by merc/ PMCs?

Wat?

no longer pemitted by international law.. and will be phazed out for the general population by 2020

Nobody's taking away my black polos.

Since you were in the ad you've been grandfathered for all colours.

What? why?

Because of these:

http://www.douchearchives.com/douche...1301650124.jpg

Find old archived copies of 'Soldier of Fortune' or any other military history magazine. YOu'll find the weirdest pics...

I wish I still had this cover, there was this musclebound guy, with a black beret, mirror shades, and an olive drab t-shirt, blazing away with twin machine pistols. It was circa early 80's. Total action hero stuff, I kid you not. So check out old military magazines. At least you'll get a laugh out of them, and get some inspiration.