Around 10dGH is good but they will be OK anywhere from 5 to 20.
You should not have goldfish and platies in the same tank ever.
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.
no. you should have 10gal of water per goldfish. you shouldn't have goldfish in a 5gal tank at all. they are huge waste producers
NO
an aquarium or a fish tank
Once a week :))))) i have experience
It's a possibility, but a goldfish and a platy should never be in the same tank as they are not compatible, and their needs are different
Yes
No, first of all they should not be in the same tank, and secondly, goldfish are egg laying fish so they do not get pregnant
Small or Big it should be kept in a huge tank with Filter and Oxygen 10 gallons per Goldfish At least
Fantail goldfish are a hardy breed, strong swimmers and they compete well for food with the ryukin, shubunkin, comet or common goldfish; making these goldfish breeds good tank mates for your fantail goldfish.
Depending on what your sick goldfish has, it will either be specific to that fish, which you should place in a separate tank to treat, or it's something that will spread to other fish in your tank, in which case you will need to treat them all.