IDW's Dead Universe? And where should I start, to catch up? (spoilers)

[Paulbot]

So the Galvatron in there is different to the one that Unicron turned Megs into in TF:TM?

Yes. In the IDW universe Galvatron, Scourge and Cyclonus are older Transformers (older than the regular cast) and never had anything to do with Unicron. The events of TF:TM (and the rest of the cartoon) have not and can not occur in the IDW Universe.