The MPU-9150 spec states it consumes 10ma. If you only have 50ma from the Mega then you don't have enough to run eight chips.
Each device on an I2C bus needs to have a unique address. The MPU-9150 only has two possible addresses it can be set to, so you cannot have up to two on a single bus. The PCA9544 would be sufficient for eight chips / two on each bus.
The PCA9544 or PCA9548 state that they both include level translation; the PCA9306 wouldn't be needed. Not sure I understand the doc's instructions on how to set that though.