The P2 and Photon 2 now support WS2812, WS2812B, and WS2813 NeoPixels! All you need is Device OS 5.3.2 or later, and Particle-NeoPixel 1.0.3 and you can blink strings of LEDs to your heart’s content.
The RTL872x platform can only use the MOSI pin of SPI or SPI1 (also known as D2) for NeoPixels because it needs to use the DMA controller in the SPI interface to output the pixels fast enough.
Sort of. It was originally forked from the Adafruit NeoPixel library in 2014, but since then the two have diverged, so there will likely be differences.
In theory, the same technique that was used to implement NeoPixels could be used to modify the FastLED library to work on the RTL872x, but as far as I know, no one is working on doing so at this time.
Understood - I have been using FastLED on Arduino and it is much easier to work with to create dynamic effects - more likely to have a cheap arduino device as a sidecar processor/driver to run an LED display than use a P2/Photon2.