The rule is 1 inch of fish per 1 gallon, so that means you'd need at least a 10 gallon tank for all of the fish you want. A glass 10 gallon kit only costs $30 at Wal*Mart... that includes the tank, lighted hood and filter. You should invest in that so you can keep the fish you want (6 guppies and 3 or so neons) comfortable and happy! There is NOOOO way you could keep more than 3 fish in a 2.5 gallon. I have 1 Betta in my 2.5 gallon tanks. The box probably means you "could" keep 2 Guppies in that tank, not 6

Please do check out the website Jackie posted, and do tons of research about Guppies, Neons, and setting up a tank, and you'll have healthy happy fish.