It probably belongs in Project Guidance, actually.
I recall the old ATtiny85 doesn't have hardware I2C so that and the FastLED library will fight over resources. They may even want to use the same timer, as both I2C and WS2812B communication is time sensitive.
Start by making this work on a regular Arduino (the Pro Mini isn't that much bigger and about the same cost as an ATtiny85 anyway) first. Then try to get it onto an ATtiny.