I have no idea what you're suggesting with a FIFO. It has nothing to do with his problem that I can see.
Serial comm works great (many PICs have a hardware serial module built in). I2C and SPI are other ways to interconnect 2 controllers or interface a controller with a peripheral designed to use that sort of communication. Serial will even allow you to connect to a PC. We do this all the time.
For communication between 2 controllers, usually serial. You don't need to use an RS232 transceiver in most cases, just connect the TX of one controller to the RX of the other (one wire) and configure the hardware serial module.