I'd handle this the same way, minus the summoning rules.

If you have an ancestor spirit like Sir Stuart, make it an Aspect you can invoke. AS far as multiple ones, I'd say it's possible but unlikely, given the nature of how ghosts are created and stick around in Dresden...of course, a sort of "Spirit Council of Elders" is a really cool concept.  If you have refresh to burn you could make them an Item of Power.

Instead of Summoning Rules, I note that Mort gets tired when he directs ghosts on-the-fly and the prevalence of thaumaturgic-level magic with the speed/method of Evocation. This tells me that you can pretty easily model what he does as Ritual: Ectomancy, and Superior Ectomancy, effectively using "Ectomancy" as an evocation element in combat. Availability of ghosts to do this is handled simply with compels.

Spinning off the idea of a "Spirit Council of Elders," here's how I might build a "21st Century Shaman" ectomancer at Chest-Deep: