First... listen to Boo... do not take advice from a 'big pet store' they are there to sell stuff... not make sure you have the right fish.
Tropical are not harder to keep than cold water it is exactly the same, you just add a thermostated heater to the tropical tank. If anything coldwater is harder as people tend to just whack a goldfish or tree in and they are massive waste producers so need a powerful filter and constant water changes.
60l is kind of small, either go smaller and keep a single Siamese Fighting fish or aim for 100l and have a much wider choice.
Do not get neons, they need a well established tank and will barely survive in a new tank. Mollies Platties and Guppies are good starter fish but don't save any young fry as you will soon be over run.
No well run tropical tank needs salt of any kind... 'tonic or otherwise.
Think I covered all the points above lol...
This is an excellent forum for biginners and experts alike... go and have a good read on there.
http://www.fishforums.net/
Lisa x
ps....goldfish need ponds not tanks... they should grow to at least 12'' and won't do that in a tank :0/