I, too, think Pierce will do a fine job. Horace is not, as already pointed out, "blue collar." He's a textbook petite bourgeoisie. And most important, he's funny. Simmons would (I think) be far too butch and lower class for the role (and he's never particularly funny, either). If the audience thinks that he might actually hit (and physically hurt) Cornelius, Barnaby, Ambrose, or Ermengarde, the comedy flies out the window.
It is a bit odd that the actor playing Horace is 13 years younger than the actor playing Dolly, but I'm sure hair and make-up will solve that.