a)Cycling a tank with many fish will produce a lot more waste, which will be stressful to your fish, resulting in higher die-off and greater susceptibility to disease.
b)Cycling with a large number of fish will increase water problems incurred during the cycling process.
c) Cycling with a lot of fish can contribute to a foul smell coming from the tank.
Every couple of days, do a 10%-15% water change, buy test kits for Ammonia and NitrIte, or, take a sample of your water to a fish store to get it tested. Most pet shops will test fresh water for a minimal fee If the store you got the fish from won't check to see if there is another local store that will. At this point, your water should test with high ammonia and maybe a trace of nitrite. If it isn't, don't worry. Just give the tank time. The cycling process usually takes four to eight weeks.
By this time, your ammonia and Nitrite levels should be acceptable (about trace levels), and you can add more fish. Do not add more fish until the ammonia and Nitrite levels have both dropped. Remember to add new fish a few at a time to prevent over-stressing the filter. If you add too many at once, your tank will have to cycle again, yet if you add a few at a time, your bacteria growth rate will just increase for a short time, with minimal effect on your fish.
====
Your tank is probably almost finished cycling, first off don't add any more fish!! Second test the water if the nitrItes or ammonia are over 0.5 then you should do about a 25% water change. I reckon the cycle could nearly be over but it may be too late for the guppy I'm afraid. Guppies are very sensitive to water quality and high nitrITes (probably after 3 weeks ammonia is decreasing to about 0) will have killed them. Its not all doom and gloom I reckon that by doing the 75% water change you saved the rest of your fish for sure.
Keep us posted on how you get on and the test results.
P.S. Welcome to the site! Sorry you're first post is an emergency though (I'm moving it to emergency forum
)