I’ve never tried the WiFi code with WS2812b, but I would expect incompatibilities due to their timing requirements. It looks like FastLED has some native support for controlling WS2812 w/o using interrupts… see here:
https://github.com/FastLED/FastLED/wiki/Interrupt-problems
Or you could look at Paul’s library that does the same thing:
https://github.com/PaulStoffregen/WS2812Serial
Anyway, I just tried compiling the V2 code for Teensy 3.2 using Arduino 1.8.9 and it worked fine (FastLED 3.3). What went wrong for you?