mix colors yes
adding more than one ff, not always recomended
a mated pair will do fine, but a non-mated pair, will probably kill each other. As will 3 or 4. If you want more than one ff of the same type, you should get 5 or more at the same time, or get a pair that is doing well and living together. It is easy to tell, they will stay together, not just int he same tank, but they will hide in the same hole and stay nearby each other most of the time.
They do not like conspecifics. Unless they are schooling.