Trust the hydrometer Luke.
Yes, sanitize whatever will touch the cider to take out enough to fill your test tube and take a hydrometer after around 2 weeks. If your around 1.000 you can rack to the secondary to clear, condition, and watch.
I have not done one gallon carbonations, so I don't know the math, but I would caution to make certain the gallon jugs can deal with the pressure from carbonation. Most I have are not built for it.
I could suggest champagne bottles that you could cap, or reuse 2 litre plastic soda bottles. Just clean them up very well to get rid of any smells. They work really well with pressure and I'm using them more and more.
In Primary: Belgium Chimay clones.
In Secondary: Braggot, pale ale, end of the world white.
Conditioning: Mead, Cider, braggot, Belgium Wheat.
On Tap: Clones, Chimay Blue, Red, Porter, malted cider.
Bottles: Far, far, too many to list.