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.
I feel quite comfortable with my 20 and a 40 by my side. No rifles though.