Following the Imam...

When following the Imam, when should one go into ruku', sajda etc.? Must the Imam say the full `Allahu

It is best to follow the imam right as he begins saying "Allahu Akbar," though doing so in a way that does not make one fall ahead of the imam. This applies to all parts of the prayer. It is especially important not to finish one's first or second salams before the imam.

permissible for the follower to add "and his Blessings" to both Salaams, or must one say exactly as the Imam does?

This is generally discouraged, even for the imam.

Wassalam,
Faraz.