There is absolutely no reason small tetras and rainbowfish cannot be kept together. On top of that, celebese rainbows are among the most peaceful out there, perhaps only bested by threadfins and other less common species like gertrudae. I've kept rainbows and small tetras together for many years, as well as with delicate hatchet fish, etc. No one is worse for the wear and they get along exceedingly well.
I do agree that it's nice when a LFS person refuses to sell fish to a situation they know will be harmful, but unless they know what they're talking about, it does no one any good.