Tax categories allow you to separate the different types of products in different tax categories. For example, if you sell alcohol and books on your website, depending on your country, it's highly probable that they get taxed differently.
In most cases, you only need one tax category, applied to all your products.
So there is no point in setting access levels onto different tax categories. You can only select one tax category per product.
Also, there is no need to have tax rules when taxes don't have to by applied.
So you setup should actually be really simple:
Only, one tax rule for the USA ( I don't think that you should tax foreigners... taxes should only apply to US based clients as far as I know ), set to your products tax category, for the tax rate 7.5%, to all customers types and limited to the user groups which should pay taxes.