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So the BRB sais that when a IC joins a unit, he counts as part of that unit for all purposes.
Does this mean that I can use this when creating my army list to take special weapons?

For example
I take a chaos sorcerer in terminator armor and make him join a 4-man terminator squad. Can I now take a special weapon on the terminators because the units model count is 5?

I supose you cannot but I want to make sure.

No. The IC Joins the unit AFTER list creation, not during.