Number of Supply Services Permitted in a Building

Please see CEC 6-102,

I want to rent half of my garage, I have the existing service meter in the garage. Now I am planning to split and build a separation on dry-wall and get another service to the other side of the garage to rent out with its own meter. But I was told to get two same voltage services in one building I need a fire wall separation. My question is:

1) The code allows to have the same voltage in one building, if I buy my power from different authorities. (Buying from different authorities increase any public safety?)

2)It allows also to have different voltage in the same building, even if for 120V rated equipment you could have power from either single phase 120/240 or three phase 208/120, or at that matter you could use 240 and 208 supply for 240V single phase rated equipment. (How does this address any safety)

3)The rule specifies self contained occupancies are permitted to have different services in the same building. What is the definition of "Self contained occupancy" means, is it having fire wall or fire separation between the two buildings, if there is a fire wall requirement, what is the rating. Is it half hour or one hour separation. What kind of fire rating I get from regular drywall.