guppyking
we are here to offer advice and guidance, so please dont ridicule those that give their time to your question.
the question you asked...no dont transfer anything from one water source to another without proper quarantine. the snail would probably be eaten anyway as goldies can suck them out the shell.
An apple snail requires 2 gallons of water
Now...the fantail..yes it can swim around in a 5 gallon but it cant grow as nature intended....goldfish are messy and require large amounts of water and filtration.
20 gallons is rthe minimum required for a 5 inch and over fish, excluding tail.
As your fish will be small its suggested you house it in a 10 gallon to stop water quality deteriorating fast and to allow for growth and freedom of movement.
If you do not wish to take the advice of those people who have tried to help then thats for you to decide...politely