Taxonomy

Hi

I am struggling with the taxonomy for my site about health care. I hope you can help. Also I wonder if GDV2 is going to make changes I should wait for regarding this.

Some fysiotherapists, like some orthopeeds have special focus on kids, old people or sports. So I can have Category:Fysiotherapy. Subcategory: kidsfysiotherapy. And the same for orthopedics.

But since both focus on kids it would nice to have a page with all fysiotherpists and orthopeeds having a special focus on kids.

I thought about same sub category, but but categories can only have one parent.
I thougtht about tags, but every client can add their own tag. So is there a way to offer limited tags?
I thought about custom fiels, but how do I make a page with places of a certain custom field?

This must be a common issue in directories, so I hope you can help.

Peter