Did you specify "localhost\Administrator" as the guest processing account? That will not work consistently. Try just "Administrator" (if all the local administrator passwords are the same), or specifying separate hostname\Administrator credentials for each VM.
That said, I think there's some confusion here. First there's the question of whether or not you need to use the named Administrator account. So far I've mostly seen this to be true with guests not accessible over the network (due to VIX limitations), and even then there seem to be exceptions. When connecting over the network, I have generally found it to be unnecessary; there seem to be some variations depending on particular GPOs, OS version, and whether or not the VM is part of a domain.
You do need to use an account with local administrator privileges. That doesn't mean the account has to be local, it can be domain, just so long as it has the rights of a local administrator (typically by adding it to the local Administrators group).
There are also permissions requirements for each supported application (MS SQL, Oracle, etc), but you can find those in the user guides.