How am I supposed to craft ironwood, steeleaf and thaumium turbines or other machines that require materials from other mods, if I have no content-adding mods, but GT6 and IC2?

Ironwood and Steeleaf can be made using IC2 Crops, and I think you might be able to make Thaumium using a Bath and “Liquid Mana”, if not then well, you cant make those things then. Things exist because things exist. That is how Compat works.