I would say that livebearers are sociable fish rather than schooling, but they do need company; a lonely livebearer is going to suffer and not live life to its full potential, so in a sense the Petsmart lady had a point. On the occasions when I've had to keep livebearers on their own, I feel it has been detrimental to their general wellbeing. But obviously you're not going to see the real authentic schooling effect of a group of fish behaving as one- livebearers are individuals, albeit social individuals. (Bit like humans as opposed to sheep

). I wouldn't say they "pair off" to breed, more like the male having a quick dash at it (livebearer females never seem to enjoy sex much

). But I often see livebearer females swimming around together and seemingly enjoying each other's company.
Anyway, to the schooling suggestions:
Just remember that danios are extremely fast zippy fish and need a long tank; most 20 gals may be on the short side.
Harlequins would be happier, and I would second the espei suggestion- they can be stunning looking fish. (they've got them at my local shop and I am sorely tempted....)
You may find other small tetras hardier than the neons: glowlights, possibly even black neons.
And have you thought about pygmy corys, or the habrosus corys?