Hey mate! Definitely do rebuild, I know people who've started to prefer rebuilding their OCC coils to taste and not even bothering with the RTA section.

25g or 26g seems to be the most popular, when I helped a pal just so I could get a sense of doing it we used 26g and it was... ridiculously easy. It'll depend on what you want to do and the ohms you want to reach. For higher ohms, 28 might be easier.
I have a feeling a lot of people who enjoyed rebuilding kanger BCC coils for the evods/protank 2s are going to fall in love with the nanos and minis. You still have to get the wicking right, but it's that same feeling of "oh, this makes perfect sense." I'm sure they'll mess it up and come out with dual coil versions, but yeah definitely do consider rebuilding.