I know this is an old thread, but I was also trying to get the Photon to work with a device that only operates at 300 baud. Doing some digging it looks like it’s a hardware issue with the actual chip that wasn’t designed to go below 1200. The documentation however does list communication as anything from 300 - 115200. It should be noted that 115200 is the correct maximum.
USART protocol datasheets:
Core (STM32F103)
Photon / Electron (STM32F205)