I only have 1 20 gauge shotgun am I doomed?
Not at all- I routinely recommend a 20, or less often, a 16 gauge if that's what you are comfortable with.
The list you are probably referring to was copied from an unknown FReeper- I do recommend each person have a shotgun, a sidearm, and if possible, a rifle. Preferrably of common calibres/gauges. In other words, if you have a 20 gauge, it is less of a logistical problem if everyone else in your "unit" uses it, too.
No, you are ok, but I'd lay in extra shells. I have 10 boxes of 12 ga shells here and three more in the back of my Bronco.
Don't you have a handgun? You should.
I have a Glock 17 with six 17 round mags, plus a Kimber Gold Match 45 auto with several Wilson Combat 7 round magazines.
Yes. A 22 rifle is the survival firearm.