Intrusion..!
Goldfish are cold water fish and should not be kept in an aquarium with a tropical molly. There is no way they can cross breed. Fish are not stupid and do know the difference between one of their own species and a species that should not be in there with them. It is quite possible the goldfish will eventually kill the molly. Failing that, if your tank is not heated, the Molly will probably die during the winter if the temperature gets below 65F for too long anyway.
All goldfish types are the same species, so all types can breed.
as far as i know yes,unless your fish is aggresive
Yes they are different variations/varieties of the same species.
yes
NO.
Any kind of goldfish will mix OK with any other kind of goldfish. Goldfish are all one species with many 'sports' that have been developed by selective breeding. The only No! No! is if you have some well bred fish and don't want them to cross breed at breeding time. Then you will need to keep type with type, otherwise, since they are the same species, they will cross breed and your resultant fry will be almost worthless crossbred goldfish.
Some fish show no visible differences in the sexes while in others the differences are very large and obvious. It all depends on the Genera and species of fish and sometimes also the sub species.
When goldfish are large enough, they are called carp. Tilapia is not a goldfish or a carp, although the species does have some similarity.
It is a goldfish with a big tail that looks like a fan?
OUI!