Gomi is a natural 155 pounder, and while he does hit very hard for that weight class, I don't think he hits harder than Alves (definitely doesn't kick harder). Daley might be the harder puncher (though I even doubt that, as he couldn't even rock Kos with a sucker punch after their fight and Alves rocked Kos several times in their match), but again Alves kicks a hell of a lot harder.
That said, I do think Alves might have problems with Diaz as Nick is extremely tough and Alves has seemed mentally weak lately (don't think he's ever mentally/emotionally recovered from his title loss to GSP). If Thiago wasn't able to finish the fight early or totally dominate, Diaz's constant pressure might eventually get to him.
I would love to see it either way though.