It depends on the fish, the tank and your water parameters (pH, hardness).
Rectangular tanks are easier to stock than cubes because they have more swimming length.
There are very few true cold water fish suitable for 60 litres. I can't actually think of any. Which country do you live in? I ask because UK legislation makes it difficult for us to buy any true cold water fish (they could wreak havoc to the native fish if they got into a lake or river).
Goldfish are cold water fish that we can buy but they need tanks bigger than 60 litres.
However, there are temperate fish. Their temperature can't go as low as cold water fish but as long as the room where the tank is located doesn't drop below about 18 deg C in the middle of a cold winter night, this shouldn't be a problem. There are a few temperate species suitable, eg the white clouds you asked about in another post. These come in two colours - the usual grey bodies with red fins, and a yellow bodied colour (called gold white cloud mountain minnows). These could be mixed to make it look like two shoals of different fish. White clouds can live in a wide range of hardness.
But avoid danios. They may be small fish but they are very active and need more swimming length than a 60 litre tank can provide.
Tropical gives you the widest choice. There are many species that would be suitable for 60 litres, depending on the hardness and pH of your tap water. Too many to give a list.