There is no definitive base sizes for the different monsters, use what fits.

House Rule: It is suggested to mount monsters as follows:

• On 40mm square bases: Pegasus, Griffon, Manticore, Wyvern, Dragon, Dark Pegasus, Hippogriff.

• On 25mm x 50mm bases: Cockatrice, Unicorn, Gorgon

This is up to the players discretion.

House Rule: It is suggested that large creatures be mounted as described above, and that large creatures on 40mm square bases be considered large. It is further suggested that collections of small creatures on 40mm square bases (snotlings, Nurglings, swarms) do not be considered large creatures. Monsters mounted on 25mm x 50mm bases should not be considered large

Allow figures to fight the monster if it seems reasonable, the figures do not have to be in contact with the base.

Yes.

Must cause 1 wound to freeze unit.

Yes, And the Gorgon too.

Yes and Yes.

No.

Yes.

House Rule: It is suggested that the dragon have an unmodified 5+ armour save.

Yes.

House Rule: It is suggested house rule to allow the wyvern's armour save to be unmodified.

Not covered in the rules, the following is a good house rule: If the monster does anything other than flee, then place the monster in the controlling players deployment area during the controlling players next turn. The controlling player determines location. Follow the monster reaction table as normal from there.

Yes.

It will miss the rest of the opponent's turn (hand-to-hand combat, magic), as well as the entire controlling players turn (movement, missile fire, hand-to-hand combat, magic).

Cannot cast spells, cannot dispel spells.