Type-II EPS is plenty for a residential slab- Type-IX is overkill, but commonly specified.

The ~10 psi compression rating for Type-I EPS is a potential problem for sub-slab, but not necessarily. Don't skimp on rebar or concrete thickness if using Type-I EPS under the slab- the slab rigidity becomes more important for weight distribution when you go with low-density goods. In water-saturated soil Type-I EPS can take more than a 10% hit in thermal performance until the tide goes out. As long as drainage is attended to and your slab is a nominal 4" (3.5" min in the thinnest parts) , it shouldn't be a problem.