Hello there

If my calculations are right, your tank is right around the 15 gallon mark. This can be a fun tank size to work with. It's best to have small fish in this one, but I'm sorry to say that all fish are susceptible to ich, with the exception of scaleless fish (plecos, cories, etc). I say this from experience - I once had a terrible outbreak of ich, and all my fish were covered in it except the scaleless ones.
Anyway, let's start with the top of the tank. I would put in a couple of honey gouramis. I think this is a good number, someone please correct me if you should keep more or less together. Those will stay at the very surface of the water, and are gorgeous. For the middle area, I would get a school of 6 or 7 harlequin rasboras. I LOVE these fish, and find them very pretty and entertaining.
Now for the bottom, I think you should get cories. These are the most entertaining of any fish you would be able to keep in a 15G. Keep at least 5, they like groups. If you can find the pygmy species, you have the great option of including more in your group. These can be very colorful as well, especially the julii or panda cories. If you have good filtration on your tank, I would throw in a few khuli loaches as well. These are very noctural (I rarely see mine) but extremely colorful and full of character. They look just like tiny sea snakes or eels.
To top it off with a cleaning crew, get 3 or 4 otos, and if you're feeling spunky, a shrimp or two. And there ya go! All levels filled with colorful fish full of personality, and you're not over stocked.
Depending on what you come across at the store, here are the Latin names for all the fish I mentioned (someone correct these if I'm wrong):
Honey gourami - Trichogaster chuna
Harlequin rasboras - Rasbora heteromorpha
Khuli loach - Acanthophthalmus kuhlii
Julii cory - Corydoras julii
Panda cory - Corydoras panda
Pygmy cory - Corydoras pygmaeus
Oto - Otocinclus affinis
Ghost shrimp - Paleomonetes sp.