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Here is a mod I just D/L'ed, but its only 1.6.4 for now. It lets you add any mob from any mod to dungeon spawners with configurable wight.
I added Special Mobs to my pack to give it some variety and they can be used with this mod too, but I am pretty sure that the vanilla mobs spawned in dungeons already gain random Special Mobs properties as they spawn so even vanilla spawners have variety amongst the mobs spawning. However, a skeleton spawner will always spawn skeleton types, etc...
LINK
Also check out the Limited Mob Spawner from Project Zulu, it's even more customizable, but unless you're building a map I'm not sure how to use it really...It has options to spawn mobs almost like a playlist as well as many more options. And for even more customization, check out Mob Properties if you're still in 1.6.4. Can't WAIT for Father Toast's mods to update to 1.7 ! =(