Why not? I have done it. That's why I said to blow it dry and then use some dry gas and swish it around or use some type of industrial cleaner or you could use kerosene or Diesel.
On really dirty tanks.you use soap, water and hand full of nuts from bolts and shake vigorously. Repeat till no more rust particle or dirt comes out.
Yes, just what he said. When you're done just leave it sit out for a few days with the cap off so all water evaporates, if you want to go the "extra mile" get a bottle of 90% alcohol (look at the label, NOT 70% which is most common) from the pharmacy and pour that in there, swish it around a few minutes, and pour it out. The alcohol will absorb any water droplets (up to a point of course, it won't absorb a quart) and take the water out with it.
If you want to get really anal about it, go to the liquor store and get a fifth of straight grain and use that. Besides being expensive though, what a shame to use it for that, instead of putting it to better use, like pouring into a watermelon next summer....