Correct, don't get 6" pads. Get a 5" backing plate + 5-5.5" pads. The Meguiars Thin Foam Pads [cutting, polishing,finishing] will get you through most tasks. 2 of each or better yet a 12pk. but only get 2 black pads if you buy a 12pk. because that's all you'll need.
As far as compounds you could survive by using Meguiars Ultimate Compound and Ultimate Polish.
I have that same polisher and to be honest I wouldn't even waste my time changing out the grease or changing anything about it unless you have time to kill or just get super OCD about things... It's a good polisher straight out the box. The changing of the backing plate to a proper 5" backing plate [Meguiars or Lake Country] will make worlds more difference than all the mods combined could ever do.
If you have this the next upgrade is to a Griots 15mm. not the GG6 as you'll basically be getting the same type of machine.