If this is cement I'm assuming you have hydronic heat (water/gel/oil). You can use pretty much anything you want over hydronic. In fact, some of the best results with in-floor heat is with wall to wall carpet.

The carpet/rugs aren't the issue. The underpad is what blocks most of the heat. If you have tremendous amounts of exposed flooring/concrete/heat, you don't have to worry about what you throw down in a small area. The rest of the space will be heated very easily by all the exposed cement.

If this is electric, you will need to pay very close attention to what type of rugs you put down. Again, over small areas such as a runner and a couple of 5x7 rugs, you should be fine.