13. Can a lamp of the same wattage be used with any ballast
rated for that wattage, such as, a 400 W Metal Halide lamp
with a 400 W High Pressure Sodium ballast?

No.
A lamp will only operate properly when used with the ballast designed for that
particular lamp. The lamps to be used with a particular ballast are listed on the ballast
label and in the ballast literature. Using a lamp that is not specified will affect ballast life,
lamp life or both. HID lamps all have specified unique operating characteristics.
The ballasts have been designed to operate the lamps within those specified parameters.