7D and lenses effecting cross over focus points?

It was brought to my attention a few days ago that the full/proper operation of the cross over focus points is dependent on the lens you are using. In other words the faster the lens the more of the focus points can be utilized as cross-over points. Can someone please verify this and perhaps explain a little about this for me. I am contemplating getting the 7D and perhaps the Sigma 150-500mm lens. Unfortunately it isn't a very fast lens and I don't want to be crippled with minimal cross over points.

Thanks

Wayne