wow....alot going on there.
I will try to answer what I can of the questions you asked. But first a few comments.
DO NOT add any more fish to this set up. You already have a sick fish which could be a spreadable disease, and I am sure the move was a stress as well. Second, without knowing what fish you have, you should not be adding others that might be incompatible. A perfect example is adding a red tail shark to a tank with goldfish, ...another would be adding neon tetra which prefer warm water while goldfish prefer cool water.
1) It looks like you have quite a few fish in the tank already and so you would need to tank individual or closer group pictures for us to help you identify them all. I would do this before you add any other fish to your setup.
2) I can not tell from the picture of the fantail about the fungus, but I would isolate it and put it in quarantine and treat it with melafix or pimafix. Given the size of your tank, it would be impractical and a bit unnecessary to treat the whole tank unless there are other signs that the fungus is affecting the other fish.
3) If you dechlorinated the water, the water is chlorine/chloramine safe, but you will need to use the testkit to check the parameters of the water. Post the result for us to give you advice. A few questions though: did you keep the gravel and filter from the previous set up? Did you clean either in any way?
It is important to keep the biological filtration, the useful nitrifying bacterial colony, intact with a tank of that size housing so many fish. My hope is that you just added water and let it go.
4) The plants look nice. Do you know what they are? do you know what type of lighting you have on this setup? It looks a bit dark.
5) Depending on the cichlids, I would guess they will eat your new plants, should not be in a tank with goldfish or a red tail shark, and should be kept only with other cichlids. You should get some good pictures of them and ask the cichlid experts on this forum for advice. You may be able to trade them at your LFS
for other fish or sell them on craigslist in your area.
6) A saltwater tank would require a lot more equipment to set it up and the ABSOLUTE knowledge that this tank has NEVER had copper it in. Unless you know this, it would hardly be worth trying to setup a marine tank as the copper will poison everything. IT would be too risky IMO
. Also, there are several colourful options you could consider for freshwater, such as discus. But before you jump into any changes, read up and inform yourself. discus - Google Search
8) Test your water, test your water test your water. Remove the shark and return the tetras until you decide what setup you really want. You may even eventually decide not to have a goldfish tank. Just decide what you would ideally want before adding anything else to the tank. And make sure your water is safe.
9) What type of pleco did you add? Check fish compatibility before adding!
10) Check fish compatibility before adding.
Freshwater Fish Compatibility Chart - Rate My Fish Tank