No you mook
Leaving a dead goldfish in your tank will boost ammonia levels.. You dont want ammonia in your tank. It rapidly kills fish
No, absolutely not! Brackish water has salt water in it and it will die because of that!
Goldfish are very hardy fish and can survive a wide range of water conditions. It wouldn't surprise me if they were the last fish to die after a major tank/water problem.
Angel fish are semi aggressive fish. They may have a tendancy to pick on the gold fish. Also, angelfish need tropical warm water to survive. Gold fish like cold water. I would not put them together. If the Angel is big enough it will eat your goldfish.
Well marine is sealife so yes Actually, goldfish are freshwater fish and not marine life.
No, goldfish are best with goldfish with the exception of weather (dojo) loaches, bristlenose plecos, and apple/mystery snails.Although goldfish can survive in warmer water temperatures, they are a "cold water" fish and not a tropical fish.
Yes
No, just one goldfish requires 20 gallons (75 litres) on it's own not just because of it's potential size, but because goldfish are very high waste producers
It is important to remember to put things in a goldfish tank, even goldfish get bored. Some of my suggestions for tank decor are artificial plants and a "hidey hole" for your goldfish.
Depends on the temperature
You should not have goldfish and platies in the same tank ever.
goldfish will survive in tap water but it is best to put a bit of chlorine control in the water plus you will need a filter if you want the water and tank to stay cleaner for longer :)