Just checked the latest FAQ and wanderers in the wild are exempt from the rules for Heroic tiers when taken as allies. See the last sentence below.
Quote:
Page 133 – Adding Allies
Add the following paragraph:
Naturally, when multiple armies ally with each other to fight together, they will each be led by an appropriate leader. For a Historical Alliance, each allied force must contain at least one Hero with a Heroic Tier of Hero
of Fortitude or higher. For a Convenient Alliance, or an alliance containing Impossible Allies, each allied force must contain at least one Hero with a Heroic Tier of Hero of Valour or higher. Models from the Wanderers in the Wild army list ignore these restrictions.