I hate to have to say it, but a 60 litre tank isn't big enough for even a single goldfish. I would advise that you return them to the shop. Goldfish are massive waste producers, and require huge tanks. They are also coldwater fish that require different temperatures from tropical fish.
As for the danios, they are very active fish and require a lot of swimming space, so I wouldn't really recommend getting any more.
How about a school of colourful fish like neon tetras, and perhaps some corys for the bottom of the tank?
To give you an idea of how the tank could look, here's a picture of my 60 litre planted tank:
In the tank pictured are neon tetras, a gold ram and sterbai corys.
Have you read up on cycling the tank, and do you have a test kit to check your water parameters?
And a good site for ordering plants online is http://www.aquaticplants.eu.com/