Everything can be done, as long as you respect the impedances of both input and output, the latter is very critical, and you must respect it because if you place more speakers in parallel, the impedance low and you can burn the amplifier, to connect more speakers you must use the parallel series method, so as not to modify the output impedance, at the microphone input you can connect a guitar without problems, you can even modify the input for other applications, greetings