How to safely supply power to multiple shields from single power source?

I have been given lots of conflicting information in regards to how to set up the power supply to my device, so I thought I would post here in an attempt to get some clarification. Also apologies if this seems like a simple question, but I am mainly a software guy so am not too clued up on the power side of things.

I have:

Arduino Mega 2560
SIM900 GPRS/GSM Shield
Sparkfun GPS Logger Shield
Sparkfun Wifi Shield - ESP8266

The shields will be stacked on top of each other.

Now to power these devices I have 6AA batteries wired to this UBEC to regulate the power to 5V 5A.

The SIM900 module needs a large amount of current available (up to 2A) to it during transmission, and the wiki says to power it via the external jack to allow this. The Mega, GPS shield and Wifi shield I believe will operate simply with 5V 500mA.

Now given this information, what is the best way to wire in the power to ensure that the SIM900 gets the full 5A available, and all the other shields are also safely powered?

The GPRS board has an external power option. This should be used.

I understand that, but does that mean it solely takes power from the barrel jack?
Where should I wire the power to to ensure each shield gets what is required (Vin, 5v or split the power cable and wire the SIM900 separately)?

Would it be safe to power it something like the image below, allowing the GPRS shield to be 'externally' powered via the jack and the remainder of the shields to be powered by the arduino via the 5V pin?

Or is this exactly the same as just powering them all from the 5v pin?

If you use an external 5V regulator attached to the 5V and GND pins on the mega board, then you eliminate the issue of overloading the onboard 5V regulator.... BUT, with that said... it could be better to use what you have drawn to power the GPRS board to minimize the possible voltage loss through multiple stacked headers. Think of it as possibly supplying the GPRS shield with cleaner power. Not required... but not a bad option.

Only when using the DC jack on the MEGA board would you be limited by the onboard 5V regulator and you are avoiding that with your approach.

So even though the SIM900 is stacked, as I switch the power setting on the SIM900 shield to external there won't be an issue with it being connected the 5V pin through the stack? (If that would even cause an issue?)