Ok, here is my best guess on what is all going on in there. When you slowly added the materials, it did nothing. Cycling the tank is when you build up the beneficial bacteria needed to break down the fish's waste. This is only done by the presence of ammonia in the tank. The guppies that were in the tank started producing the ammonia that is necessary, but weren't in there long enough to fully cycle the tank.
Now, ammonia and nitrite are building up in the tank, weakening the fish's immune system. This is why you are now seeing the columnaris.
Before you add any salt, do a water change, about 30%. Over the next few days you will need to do partial water chagnes. Also, take a sample of your water to the LFS and they should test it for free. I would recomend purchasing a good master test kit. Ideally, you want one that is liquid based, not the dip stick type.
Also, your tank is very over stocked. Goldies need 20 gallons for the FIRST fish, and ten gallons for each fish after. The reason why is goldies are very messy fish, and will also grow very large. Even though you just have babies right now, they will need a much larger tank in the future. I would recomend looking at ebay, since there are many good deals avaliable on there.