Dwarf puffers are a lot of fun
. Just stock lightly and try to just have one male and a few females. You will need to feed live snails sometimes to keep the teeth worn down so keep that in mind. Krill wears down teeth but not as well as snails.
There aren't many fish that are "good" tankmates for dwarf puffers. Oto cats usually work and sometimes cory cats are fine too. Both species are peaceful bottom-dwellers.
This is a great dwarf puffer website: Dwarf Puffers : Home
I wouldn't get a GSP. They get too large for a 12-gallon and are not freshwater fish. In fact, they do best in a marine aquarium by adulthood. Personally, I would stick to the dwarves. They stay small, are easy to care for, are now being captive-bred, and are one of the few truly freshwater puffers.
You don't necessarily have to cycle for two months...you should just cycle until you have no ammonia or nitrites for a few days in a row. The amount it'll take depends on the individual tank and cycling method. I knew someone who's tank took five months to finish cycling...I'm not kidding o_o. That's rare...just like the person who did fishless cycling and cycled the tank in a week.
If you want to cycle pretty much instantly then pick up some Bio-Spira. Don't let stores sell you other cycling products...they rarely do anything. If you don't use Bio-Spira then you could do the fishless cycling method of adding a small amount of fish food every day or so and monitoring the water parameters.
Make sure to get a good test kit...avoid the strips.