I don't have any practical experience with this yet, but I would try doubling the PCs number on a first encounter, and every group of tertiary PCs encountered should probably have a secondary PC leader unless the scene isn't that important to the overall plot.
Remember also that armor is armor. Two tertiary PCs without armor might be difficult but doable for the Lady in Waiting to dispatch herself using improvised weapons, but two mercenaries in full armor are too much.
With the fantastical you'll just have to figure that out as you go. Test them against one Other first, and then threaten them with an entire squad when you want them to choose the better part of valor.
The last thing to remember, which applies in any role-playing game, is that how the adversaries act is often more dangerous than their numbers, armament or special abilities.
Back alley thugs can simply make a frontal assault, or they can try to guide the characters to a prepared ambush, another smart tactic is that the thugs can make extensive use of range attackers while their own melee fighters keep the PCs' strong fighters at bay (utilizing defensive options instead of offensive ones), or knockout poison gas. It depends on the resources of the main adversary, but almost anything is possible when dealing with people.
Even "dumb" animals rarely act stupidly. A shadowcat should circle the party before trying to go for one of the easy targets, (most often while hidden beyond sight), which you can rationalize as it smelling the most fear from one of the non-combatants, not smelling of oil and metal, whatever. Also, it'd try to drag the person off from the group, it wouldn't assault a lone person in the middle of the group.