You'll be a master at building your own coils in no time on the Subtank, and you'll be saving tons of dough doing it too (I started rebuilding back in the Protank days, and since then moved onto to Kayfuns, Foggers, Orchids, Buddhas, and most recently my new Subtank Mini and Goblin Mini). You've probably already heard the deck on the Subtank is a breeze to build on...probably the easiest of any RTA.
You'll be fine w/28g (although I expect you'll be experimenting with various gauges later). The first build on my Subtank Mini which I received a few days ago is 28g @ 3mm...for me it works great.
By the way, not sure which cotton you purchased but I bought this bag of Japanese cotton (Puff brand) around July of last year and it is primo. I still have plenty to last me at through next year even after sharing some with fellow vapers:
Amazon.com : Cotton Labo ORGANIC Cotton Puff Size M (200pc) : Cotton Balls : Beauty