Looking at the Hunters and Mercenaries, they have an individual skill rating, it does not vary based on number of hunters, yet they still cannot take Strain. So they are used as individuals. They are a type of encounter called a Rival (which is explained in the main rules). Rivals act individually, take damage individually, but cannot take Strain.
As far as Minions go, you don't have to run Minions in groups, you can run them individually, but they are so useless individually, there is usually no point. If you do group Minions, then since they attack and take damage as one blob, moving them apart just causes more trouble then they are worth, the whole point of using them really is speed.
A good Rule of Thumb to try with any published adventure that uses Minions: Either don't use Minions and kick them up one notch to Rival (or whatever the system calls them), or double the stated number of Minions used. Adventure writers and game designers always think Minions are more dangerous then they are, and lowball the numbers.