I'm getting two Finches, BUT..... ?

...But I've heard that you HAVE to get Finches of the "same species", for example: 2 Cordon Bleu or 2 Lady Gouldian. Is that true? I've also heard that 2 boys might kill each other and so it's best to get 2 girls. Is that also true? Are girls as colorful as boys? I don't really have a preference between boys and girls; I just want them to be colorful, happy, and get along with each other!