You have two possibilities of powering the shield according to the adafruit documentation.
A: power the Arduino over the power plug (not the USB plug) of the Arduino and set the VIN jumper.
B: Leave the VIN jumper off and power over the power connector of the shield.
Don't try other possibilities. They may kill the shield or the Arduino.
The reason ...
We cannot say, what single board you need to use. There are many boards, that are fitting to your requirements. You yourself have to choose.
Most of your listed boards use the same microcontroller (as Juraj wrote in his comment):
The Arduino Uno (doesn't matter where you buy it, it's the same) and all the boards with a 328 at the end use the Atmega238P. The ...
I guess your problem is, that you are using SoftwareSerial. Doing Serial in software means, that you can only listen at one interface at a time, thus only one of the fingerprint sensors work at a time.
But you are using the Arduino Mega, which has 4 hardware Serial interfaces. They are named Serial and SerialX (with X being 1,2 or 3). You can give a ...