Using GSM shield with mega 2560, wiring instead of stacking

Official Hardware Retired GSM Shield
1

I got my GMS shield working stacked on an Uno, then got it working with six wires connected instead of stacking. (Gnd, 5V, Vin, Tx, Rx, Ctrl->7). The application is for sending and receiving SMS messages, no other functionality needed.

When I try to get it working via wiring on my Mega2560, it won't initialize. I'm using an Adafruit SD card shield, so digital 10, 11, 12 and 13 are in use. I've tried using 14 and 15 like this:
#define TXPIN 14
#define RXPIN 15
#define RXINT 14
with no luck.

Unlike the Uno, this Mega has a separate 5V power supply, so instead of connecting 5V to the Arduino's 5V, I'm connecting it to the external 5V power source. (All share a common ground.) I have the CTRL (reset) line connected to Arduino digital 7.

Is it OK to use RXINT and TXPIN on the same line?

Should the GSM's VIn be connected? If so, to the Arduino's Vin or 5V line, or to the external 5V?

2

Fixed, at least for now, and I honestly don't know why. In case it helps somebody else, here's the configuration that worked:

WIRING
Modem Arduino Mega 2560
GSM TX digital 50
GSM RX digital 51
CTRL digital 7
VIN not connected
5V 5V
GND GND

Software setting in the AVR_ATmega2560 block:
#define TXPIN 51
#define RXPIN 50
#define RXINT 51

This setup uses the Arduino's 5V out as a power source, but it appears to be enough to send and receive SMS messages.