Bela OUTPUT is fine in a guitar pedal chain. A series resistor of 470 ohm to 1k might be a good idea to reduce stress on the audio CODEC in case of a short circuit or ESD, but this has a small effect on noise performance (adding resistance makes the line MORE susceptible to noise, not less).
Bela INPUT is more of a concern for guitar because its input impedance ranges from 20k to 100k, depending on the PGA setting.
I think you are fine with a simple JFET buffer on the input and nothing special on the output.
Concern about impedance matching has its roots in transmission line theory. With cables that are long compared to the wavelength of the frequencies you are transmitting, an impedance mismatch will result in reflections (return loss). At audio frequencies this effect doesn't become interesting unless you are transmitting audio several miles across a telephone line.
Generally you want to keep output impedance as low as possible so noise coupled onto the cable has a hard time generating audible voltages on the line.