Confusing I2C connection and Power Problems. PX2.1

I have problems to connecting Pixy IR lock to PX2.1 and Power problems.

The Pixhawk v2 Feature Overview PDF is saying that.
I2C - 4 pos (1 fitted as a stand alone, I2C_2, (old internal)
Old internal that means i shold connecting pixy to the gps/compass I2C because it is external i2c?

If i use LedderOne lidar and PixyIRLock MP is showing me that one or the other is working. They wont work at the same time if i pull out ledderOne i get bad lidar health but the bad vision position message is disappearing.

The power problems i get is when i connecting PixyIR lock to the PX2.1 i saw that the voltage is getting under 4V and pixy crashes most of the time after that.Sometimes even in bootup(BatteryConnecting)