Seems like, if three player tombstones are enough to summon skellies, three monster corpses should be enough, too. I'm currently at 56 necro, and I only use skellie warriors with a few armor mods to put them above 600. Archers, I might pull out if I am going back to the crypt or mushrooms, but they're just too fragile. Mages? Never. If I could summon them back from three monster corpses, I'd be a bit more likely to switch it up.