Could I keep 7 or 8 neon tetras in a 5 gallon tank?

According to the fish tank rule of thumb, 1 inch of fish per gallon. You would about two inches too many. But then again, neons are very hardy and won't mind a few buddies more.

As for the water changes, 25% is a good amount. More than 25% and you can change the water parameters and accidentaly kill them.One question though, do you have a filter? A filter would be necessary considering you are overstocking. If you do have one, a 10% water change per week will do. If not, go with 25% and get one.

Nothing else in the tank is also a bad idea. You need at least 1 inch of gravel on the bottom (or sand which is harder to keep). The gravel helps bring out the color of the fish if it is dark and and can be used with an undergravel filter. You should also have at least 1 plastic plant for the fish's mental well being. How would you like to be in a bare room painted white with nothing inside?

Follow those directions and do not overfeed and then your fish and you will be fine :)

That's great. Are you making these your first fish? You should put in only 3 neons at first if you are first doing the nitrogen cycle in a new tank. Neons aren't usually good first fish to start the cycle but it won't hurt the fish. It will just take longer. Make sure you don't overfeed the fish so the tank doesn't get too dirty.

Perhaps 1 or 2 male guppys and 1 more neon

7 or 8 is fine. yes they need to be in a 'school' to be happy so no less than 5 having a few over the minimum ensures the schools survival should 1 die.

As long as you keep up with regular tank maintenance (water changes) this should not be a problem.

That would be fine.

Neons are very thin fish, so you have some wiggle room with the inch per gallon rule, its not like they are gourami's, y'know?

I think by "nothing else in the tank" she means no other fish

That sounds good to me but no more than 8

yes that's fine but do 50% water changes

yes, i have 4 in my tanks