The reason people say keeping guppies and neons together is not possible is that they need different water chemistry values. Guppies want hard, alkaline water, whereas neons prefer soft and acidic. Neons kept in hard, alkaline water tend to be short lived (living only a year or so, instead of anything up to 4 years) while guppies kept in soft and acidic water are prone to fungus and fin-rot.
In theory at least, you could keep them both at in pH 7, moderately hard water without much problem.
Cheers,
Neale