Keep in mind that you don't *HAVE* to thin any paint. The reason for thinning is primarily so that it will go through your airbrush without having to use 100 psi to get it through. I like thin paint so I can use low pressure. The downside is that the thinner a paint is the more difficult it is to get it to cover properly and not run.
That said, if the metallics are spraying through your airbrush at the pressure you want to use, then you don't need to thin them. If they won't spray at your desired pressure or clog up your tip then they are probably too thick.
Most of the metallics I've used will spray right out of the bottle when it's new. After a while the paint tends to thicken up in the bottle and then I start having to thin it.