No, i don't mind being challenged at all.

Discussion is always welcome, right?
Anyway, first, I don't believe hard and fast rules in this hobby. I merely use some suggestions as general guidelines for each individual case as a starting point. For that matter, I personally go for 1 gal of water hold 1 inch of fish and I ALWAYS shoot lower than that. In my opinions, this hobby is about keeping fish and plants healthy and beautifully, not keeping as many as possible.
For a newbie, in this particular case, she's a 20 gal tank, which is a small tank, and as we all know, smaller tank is harder to manage since the smaller the water body, the more unstable it is. As she's just started off keeping plants, from my experience, its safe to say it will take a while for her plants to thrive, plus the tank isn't established, and without fish in there, even if she add Commercial Bacteria Product everyday for a week, or even longer, I can safely assume that the tank is still NOT established.
Gathering all the critiria, I have to conclude that 20 neons is definitely too many as introducer to this particular new tank.