I need a conversion of liters to gallons - how many inches or centimeters is it, and I can convert.
You are speaking of fancy goldfish, right? the kind with eggshaped bodies with wide fan tails? If so, they are coolwater fish amd should not be mixed with anything tropical. There are dojo loaches (also called weather loach, cause they swim around when there is a weather change) that can handle cool water. Make sure any plants used are for coolwater as well, such as anacharis.
Other fancy goldfish (lionheads, bubble-eyes, etc.) can be mixed with fantails, but the common streamlined comet goldfish should not be, as they will get to the food faster than the fancies.
Goldfish do produce a large quantity of fish waste, and need excelllent filters and water changes. So you want to make sure there are not too many in a tank, and that the tank is a good size. Don't overfeed, even though they will beg for food all the time
Good luck and enjoy your fish!