I would say no just for the fact I wouldn't recommend EVER putting tropical fish with goldfish.
That said, a twenty gallon tank would be just the right size for 2 goldfish and nothing more.
You said you treated with Melafix but did you try Pimafix which is specifically for fungus?
Did you test your water?  Usually fish do not get compromised by infection unless the water quality is off, which causes stress which leads to illness.  You need to always test your water before adding any new fish and after too!  Did you try aquarium salt?  Aquarium salt is great to use on new fish but you should always isolate them in a quarantine tank for at least 2 weeks.  I use aquarium salt when I first get new fish because it increases gill function and then it also gives the new fish some electrolytes which can help with the stress they are under coming into a new environment.  I guess there is a medicatd salt available but I have never seen it or used it.
Goldfish and tropicals present different diseases too which is another reason why they should not be housed together.

If you ditch the danios and snails and guppies your tank would be ok for 2 goldfish IF the water quality is pristine.
Ammonia-ZERO, nitrites-ZERO, nitrates-20ppm is ideal but no more than 40 ppm and ph 7.2-8 (akaline) AFTER putting in new filter media rinsed in tank water and after doing about a 75% water change.
Then after you add the new goldfish I would put 1 TBS of salt per 5 gallons of water in the tank.
You can also treat the new fish with melafix as a precaution.  It's a great tonic for adding new fish., directions are right on the bottle. I would also add a water condtioner as this helps protect the slime coat of the fish and I would also add a bacteria booster right to your new filter media.
BTW, what kind of filter do you have?  If it's just the 20 gallon that came with the aquarium then that is not big enough.  You'll want to upgrade to a 30-40 size filter.  Goldfish are extremely messy and need more water turned over GPH.  Also, you will need to provide extra aeration in the form of an airstone connected to an air pump.  It doesn't have to be big but just so a nice gentle flow of bubbles rise to the surface of the tank and burst.
Live plants are also nice.  The goldfish will nibble on these and the plants will help absorb some of the wastes in the form of nitrogen.
You also need to vary your goldfishes diet. Pick a good staple pelleted food such as Omega One or Aqueon.  Feed bits of oranges and peas and chopped spinach or lettuces.  Also, frozen food your petshop might carry.  Freeze dried shrimp and bloodworms are also tasy treats.
Good luck and if you have any questions then re-post here and someone will try and answer/help you!

Oh brother.
Well see if you can get your hands on some Pimafix asap.
If not you'll have to bring out the big guns.  I use Rid-Ich+ with great success.