Will it be possible to use arduino (im thinking either uno or leo) to use as my controller? anyone ever tried it? Thank you :)
It will work, send MIDI commands to RPi over serial UART. Check that voltages are compatible first though! Arduino out is 5V afair and RPi in is 3.3V max. A simple resistor voltage divider would do
Arduino UNO has a serial converter (atmega8u2/atmega16u2) but indeed it is a MCU.
You can use arduino with atmega8u2 or atmega16u2 serial converter to set as "MIDI device" installing firmware MOCOLUFA on it (I think there are some other firmwares to do that).
For atmega32u4 boards (MICRO, PRO MICRO, LEONARDO, ESPLORA, etc, you can install "teeonardu". This is a kind of fork of teensyduino, IDE addon for teensy boards that let you select what kind of device is your teensy (keyboard, MIDI, mouse, etc.)
Also recently I tested usbmidi library with arduino 1.8.1 IDE but found some issues.
My favourite method is MOCOLUFA for atmega16u2 serial converter (Arduino UNO, MEGA, DUE) and teeonardu for atmega32u4 boards.