Running Atmega1284P at 3.3 and 8Mhz
Atmega1284P 8MHz @ 3.3V?

Yes, you can. Documentation states this is permitted voltage for that frequency:  0 - 10MHz @ 2.7V - 5.5V http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-42719-ATmega1284P_Datasheet.pdf

Nick Gammon Arduino bootloaders?

On this page http://www.gammon.com.au/bootloader it states Atmega1284.
In code https://github.com/nickgammon/arduino_sketches/blob/master/Atmega_Self_Read_Signature/Atmega_Self_Read_Signature.ino
there is signature for Atmega1284P so it should work without issues.