'Speak evil of' is a very old-fashioned expression. To begin with I thought it was a mistranslation from Spanish, but lo and behold when I did a quick Google search there were some hits. But they are all, without fail, from the Bible. It is not a phrase we use any more. 'Speak ill of' is also somewhat old-fashioned. You only really hear it in the phrase 'to speak ill of the dead' (say bad things about someone who has died.) A more common way of saying it would be 'to speak badly of someone.' Or in more colloquial language, to 'bad-mouth someone.'

Yes. They are both formal.