landlord's letters - experiences

Just trying to find some info, and I wondered if anyone had experience of this - is it fine for the letter from the landlord to say "when *** arrives, we will add them to your tenancy agreement as a joint tenant". Has anyone done it this way, and has it been fine? Didn't know if there'd be a problem with the wording, as I think landlords usually draw up a brand new agreement when someone moves in. Thanks to anyone who can help!

The other thing - is it fine to have the letter addressed to the sponsor?

I asked my landlord to write a letter saying:

1) He is aware my fiancée will reside with upon approval of her visa. 2) He agrees for this to happen. 3) This happening with not breach my tenancy and 4) he plans to extend my 6 month lease as long as I do not breach the tenancy.

Thanks!