Minimum number of neons is 8; any lower and you are likely to experience problems but some people get away with 6 or so but that isn't the point, if you want to keep neons give them as many other neons as posible and if you can't fit 8 in your tank i advise you don't get them.
The main rule with guppys is to have at least 1male per 2females minimum, either that or all same gender groups i.e all male or all female although with all male groups some people experience alot of squabbling between the males and also all-female groups don't protect you from the onset of fry as the females can store male sperm in them for up to 7pregnancys.
Also with guppys i sugest you be prepared for fry as they are known to reproduce on a regular basis and produce lots of fry so i would go out and buy some fry food and a breeding net just in case(better to be prepared).
What other fish do you have and how many and the tank size they are in?