Yes.

No, these restrictions are not applied to allies. This means two things:

1. The allied army is not a mini-army unto itself with restriction on 25% allied war machines, and 50% allied characters, etc.

2. The components of the allied army are not counted towards the percentages of the overall army. If the ally army contingent contains war machines, they do not count towards the 25% restriction on war machines for the entire army.

Yes. So, for example, no more than 1 Swordmaster of Hoeth unit could appear in a High Elf ally contingent, a unit of Halflings would have to be present in an Empire army to take a Halfling Hot Pot Catapult, Rat Ogres could not be taken without Packmasters, etc.

No. So, for example, a Chaos Army does not have to take a mandatory unit of Chaos Dwarf Warriors if they take Chaos Dwarf allies.

House Rule: Limit allies as follows: For every allied unit of rank and file troopers taken, 1 character and 1 war machine may be taken.

House Rule: The Warhammer Design Team suggest that allies not be allowed unless your opponent agrees to let you have allies in your army.