Can't I just run my keyboard through a guitar amp?

While it is possible, we really don't recommend it. Guitar amps are really only designed for the relatively limited range of frequencies produced by guitars. You will be able to run your keyboard through it, but it's likely you will damage the speaker.

Can I use a microphone with a Roland keyboard amp?

Absolutely, Roland keyboard amps feature a dedicated microphone input so you can sing along as you play.

Can I play a guitar through a Roland keyboard amp?

Of course, an electro-acoustic guitar will sound great through a Roland keyboard amp and an electric guitar will also work. Although the limited gain and EQ options may restrict your sound, it won't harm your amp in any way.