I don't think the ghosts would go insane from use or overuse, all things considered. That said, the implications of "Ghost Story" mean that you can opt to introduce Compels or other complications based on a conception of ghost fatigue, ghost memory consumption, etc. The ectomancer's High Concept or another Aspect may provide a clue as to his relationship to his spirits, the way he tends to use or abuse them, which should in turn guide how they respond under his rule: Compelling that Aspect becomes an opportunity to customize his ectomancy experience.
Ghosts are likely to get chewed up and fade away more quickly under a Machiavellian ectomancer who sees them as cattle, cannon fodder, etc., and he may be Compelled with complications like "your spirit succumbs to the oblivion of complete apathy as he spends his final memory to hold off the rampaging ghouls." More sensitive ectomancers can be Compelled with complications like "your ghost friend is seriously fading, and you can tell he's spent his memories recklessly fighting for you - maybe you can work on resolving some of the issues from his former life in the hopes that he will take strength from that".
Having allies is a function of Resources, and summoning them using Rituals is basically substituting a Rituals check for the Resources check. There are two major thoughts on Allies/Summoned creatures/minions:
1) Allies are GM-created, and he assigns a Resources/Rituals difficulty which has to be met in order to secure them for a scene, and adding shifts to that roll secures the ally for longer. Rent-a-guard? Resources 3. Kincaid? Resources 12. Demonic assassin? Thaumaturgy Resources 35. They all *can* be secured using similar rules.
2) Allies are player-created; the GM reviews the powers and skills of the ally, and determines a Resources/Rituals difficulty which has to be met in order to secure them for a scene, and adding shifts to that roll secures the ally for longer. This is what Sanctaphrax, UmbraLux and I were trying to do long ago, and we tried to factor in the Skills, Powers, special Aspects, competence, and obedience to get a final Summoning DC used to determine how many shifts a Summoner would need.
Mind you, a ghost can be used a special effect for even Evocation/Channeling, if narrated properly.