Be prepared to make a lot of ad-hoc rulings if you use the Mob template. Many possible interactions are not very well defined (or not addressed at all) by the template. Any unusual tactics on the part of the players (or unusual abilities on the part of the creatures in the mob) can generate questions that the Mob template doesn't answer, or doesn't answer in a way that makes sense.
A Mob of Undead Ghouls for example - technically it's a CR8, 30 HD undead and would be impossible to turn. Yet the individual creature who make up the mob are all 3 HD undead. It makes no sense that because they are acting as a group they suddenly become immune to turning.
The template also states that if you take out any individual creature in the Mob the Mob as a whole gains 2 negative levels, and disperses when it has 30 negative levels.
So if you can turn individual creatures in the Mob, it would gain 2 negative levels per creature your turn, and break up after 15 of the ghouls are turned. Do you then have 15 individual ghouls left, or are they all gone at that point? (Question remains even if it's another type of creature.)
The Mob grapple ability is also not defined very well. Mobs don't have to make attack rolls to do damage, but what about grapples? Can it grapple everyone it swarms over, or does it have to stop when it grapples the first target? The template doesn't say one way or the other.
