I did not need any additional app to use an USB keyboard. I only needed an USB OTG cable for that.
But note: You can’t charge your FP2 while using an USB keyboard.
I don’t have a Bluetooth keyboard, but that seems to work without any additional apps, too.
I cannot say anything about energy consumption of an USB vs. a Bluetooth keyboard. Maybe there is a forum user that has both types of keyboards and can compare it experimentally.