Why they say that a 220ohm / 470 ohm resistance is ok?
It offers a good balance between protection and not cutting the drive voltage down too much. Mainly from imperial experiments.
Powering from both sides is always preferable if possible.
I choosed 220ohm for esp8266 + logic level converter from 3.3 to 5v, was I wrong?
You should always have the resistor the 5V side of the of the converter.
I am not convinced that those I2C converters are fast enough for Neopixel driving.