The fluval 2+ will handle a stocking of up to 24 gallons.
The volume of the tank is about 10 gallons. So he could get about 10-15 inches of slim bodied low waste producing fish in there.
The neons produce minimal waste, they should be ok as the 10 galon will be a nice space for them.
The pygmy cories will be fine also, as there is a nice surface area of space in that tank.
The guppies get about an inch, and will occupy a large area of the tank. You may have problems, but if your lucky, you got some calm guppies that arnt aggressive. You should have more, but if its working with the 3, then your fine.
Honey Gourami's should be fine too, they'll have the top of the tank. Dont know if theyll appriciate the strong current of a fluval 2, but it should be ok if you direct the flow at the side of the tank.
Go for it. Make sure the tank is cycled or you'll have problems.
Mark, whats wrong with his stocking list? Can you explain?
EDIT: Rephrasing 15-20 to 10-15.