Spirit of the Century does something very similar with rules for Minions. Each Minion has a quality on the Fate ladder, and as an individual uses that quality (Fair (+1), Average (+2), or Good (+3)) for all tests. A minion has a number of stress boxes equal to it's quality rating and do not take consequences, they just get taken out.
Minions also group up, getting between +1 to +3 depending on the size of the group, so a massive group of good minions acts at Fantastic (+6), making them a substantial threat. Finally, damage is applied to them sequentially, and unlike PCs all health boxes are filled in. As each minion gets taken out the group shrinks (duh I suppose).
Finally minions can sacrifice their independence to attach to a named NPC, adding their group bonus of +1 to +3 to that NPCs skills and taking damage in their stead. This is my favorite minion usage since it turns that Machiavellian planner with average combat skills into a viable threat so long as his minion buffer lasts.
Speaking from SotC experience it's very important to continually refer to the minions as a group, and to encourage the PCs to do so as well. Also, remind them to describe their actions in an exchange as a series, rather than a single discrete action. That way you don't have to retcon "I punch the ninja" into an attack that can take out three people, unless you're dealing with a PC that can conceivable punch a guy so hard his flying corpse knocks his two nearest friends unconscious.