You forgot one - Don't try to deliberately confuse the player by changing money into different limited use currencies.
How many games have you seen, especially on social networks and mobile, that have gems or something that you use to buy items with? The reason is easy to see: people stop thinking about money and start thinking gems, only thinking about their purchases when they go to buy more gems.
A few game have started to mix this strategy with the MMO practice of having multiple currencies, only usable in certain areas. In MMOs this is usually used to have a set list of items that can be bought in an area but to limit gaining those items to those who invest time in the events around the area. That's fine but can be annoying when you have left over credits that can't be used elsewhere. Now imagine facing that situation after having paid for the credits rather than earning them.
If you're going to be making a free to play game then these sorts of practices are up there with pay-to-win as unethical things that will only drive players away in the long run.