Interface with a Power Isolator(SI8600)

Hi, in terms of I2C pullups, 10k or 4.7k will probably work - the board seems to have them on both sides, so you won't need to add any yourself. I2C isn't that sensitive to exactly what value you use anyway.

The 5v/3.3v isn't really an issue... Espruino can be powered off anything from 3.5-16v because it's got a voltage regulator, and most IO is 5v tolerant. To make sure, just look at http://www.espruino.com/ReferenceESPRUIN­OBOARD and check that the pins you're using don't have 3.3v written by them - none of the I2C pins do, so you shouldn't have to worry very much.