I've had them together before, and they got along just fine. There probably is a difference in water requirements, as the neons live in ponds created by rainfall, and some of these ponds even dry up during the dry season, whilst the guppies are more "river oriented". I've even seen guppies living in street gutters here, so actually, both species are very tolerant and will most likely adapt to reasonable conditions.
Bear in mind that guppies are basically surface feeders, and, depending on numbers, could steal all available food from neons, who tend to dwell deeper than the guppies. This is easily ascertained through observation, and was not the case in my tank.
Also, take all precautions when bringing new fish into an aquarium, and, if possible, start with one, so you'll be able to spot any problems early on, and at minimum cost and risk.
IMHO it should work, however keep researching!
Miguel.-