The Teensy 2.0, Pro Micro and Elite-C have the same MCU: the 8-bit AVR ATmega32u4, and no support chips so they can run the same firmware, except for having different pinouts, internal LEDs and power regulator (which is unnecessary when powered over USB).
The differences are in which bootloaders they have, so you use different programs to get your compiled firmware onto them. The Pro Micro was made for Arduino which uses its own weird numbering scheme: there is a
chart here with the AVR port numbers.
The PJRC
Teensy 2.0 (not the 2.0 ++) used to be what most people used before the Pro Micro became available at a lower price. In fact, most open-source firmwares used to be developed for the Teensy 2.0 first, built on top of PJRC's own USB firmware.
The Teensy 2.0 has a mini-USB socket. There are a total of 25 pins: 22 GPIO pins on the long edges. One pin is between the rows and two pins are on the short edge. The D6 pin in one corner corner is stupid though: it is also wired to a LED on the board so you can't use it unless you want an orange glow...
The
Elite-C was made as a drop-in substitute for the Pro Micro: it is pin-compatible with the Pro Micro except that it has five more GPIO pins on the short edge and the RAW pin has been changed into a GPIO pin, for a total of 24 pins. No power regulator and no LEDs on board.
The Teensy 2.0 and Elite-C are priced about the same (available from multiple stores) but the Elite-C has a Type C socket while the older Teensy 2.0 has mini-B.
The QMK Proton C has an ARM-based microcontroller, and you would need to run the QMK firmware. The AVR-based boards run on 5V from the USB port but the Proton C has a power regulator to run on 3.3V. This has no significance for the matrix, but you'd need different resistor values for your LEDs.
The Elite-C and QMK Proton C are new and supply is more limited than for the Teensy 2.0.
If you want to not use the socket on board, but a USB breakout board or panel-mount socket mounted elsewhere inside your keyboard, then the Elite-C and QMK Proton C also have the USB data lines on pins that you could solder wires directly to. To do the same with a Teensy 2.0, you might have to splice a USB wire open.