"In your area, what is the smallest customer that can get 400Y in their home/business"

This could be requested by a house owner, there is no minimum size, indeed is part of the work we find ourselves doing more often, People are more inclined to have small workshops at home etc.. or heated pools, and Spa's quite common - But not Cheap!

"Does every house have its earth bonded to the incoming neutral, or do you have other grounding/earthing systems in use as in the U.K.?"
Yes every seperate installation has a removable link in the switchboard between E and N, other boards feed from that board may or may not depending on what type they are. Also the Neutral point is tied to earth at the transformer.

"Do you have "split-load" panels so that some circuits can share a GFI?"
hmmm im not sure what you mean on with one, but normally the RCDS would only feed the one circuit i.e. A run form the board bathroom Laundry etc any damp area where extra protection maybe prudent. possibly a PP outside for waterblaster etc as long as there werent too many or the run was to far.

Hope that helped
Cheers Craig