i think you need to do some big water changes until your nitrite is 0. Also, I'm not sure what size tank you have, but the fish combo, as you said, is not great. Stress lends itself to sick fish. They become susceptable to ailments they otherwise would not suffer from. In addition to that The bad combos might also lead to fin nipping. Tetra will do this when they don't have a big enough school. I think I've heard similar about rasboras as well.
That being said. Here is a great website with guppy related ailments and known treatments. Salt is almost always suggested. For you to do that, you either have to remove plants and anything sensitive to salt or set up a hospital tank... Here's the gist of the salt treatment:
Salt Treatment/Salt Bath
This is a treatment that is as old as the aquarium hobby itself. Treating with salt is undoubtedly the safest way to treat fishes. While chemicals such as Formalin, Malachite Green, and copper are considered by some to be more reliable, these are strong substances to impose on a guppy and must be handled with care since they have the potential to cause cancer in humans. Using salt is especially handy with guppies, since they are built to withstand a lot of salt - they do that quite well. Salt is especially effective against fungal infections and some parasitic problems such as ich. If diagnosis is difficult, try salt. Normally the salt treatment lasts 10 days. Note: This bath is not intended as an additional treatment and should not be combined with other medication.
Days 1 - 3: add I teaspoon of salt per gallon of water, every morning and every night. (2 teaspoons /gallon/day)
Days 4 - 7: Leave water alone.
Days 8 - 10: Remove half of the tank water each day and replace with unsalted, chlorine-free water at the correct temperature.
If by day 7 you see any improvement (but not a total cure), wait until day 10 to start changing the water in the tank. Be sure to change half the water once a day, for three days at the end of the salt treatment. Also, don't wait longer than 10 days to start changing the water, as the salt itself can begin to cause problems for your fish.