Thank you for your reply. I agree with this.
Because I will have the 8 MPS-9150s, it looks like it is better to have an external regulator. I saw the web site of Arduino Mega 2560, and it said "DC Current for 3.3V Pin 50 mA":
http://arduino.cc/en/Main/arduinoBoardMega2560
for 5V Pin, it is a bit less than 1A?
Therefore, I have an idea.
How about 8 MPU-9150s -> 8 PCA 9306s, and then all goes to SCL and SDA on Arduino Mega? With EN, the channels can be switched very rapidly, like a multiplexer?
Or would it better to have 8 MPU-9150s -> 1 PCA9548A (or 2 PCA9544As), and then one single PCA 9306 (or 2 PCA 9306s)?
PCA 9306
PCA9548A
http://www.nxp.com/documents/data_sheet/PCA9548A.pdf
PCA9544A