My guess is "Rubber Man" is the son of the original owners Dr. Charles and Nora Montgomery. When Charles resurrected the child a la Frankenstein, Nora said the baby wasn't human anymore. What if he survived all these decades and was the one who murdered the gay couple?
I think Frankenbaby might have been the father of Constance's four kids, and maybe he was the original Rubber Suit Man, but whoever is in the suit needs to be a big, immediate, cliffhanger-generating shocker when he takes off the mask.
That's why I think he has to be played by one of the actors already established as main characters, but it doesn't make sense that any of the already-introduced characters
to be the Rubber Suit Man.
It makes the most sense for him to be the one major character we've yet to meet - Constance's fourth child - and the only main-cast actor who would plausibly play him is Evan Peters, on the assumption that he's Tate's twin brother.
Vivian being pregnant with twins is the clue - this isn't the first time a good/evil twin thing has happened in the house. And here's a juicy possibility - what if Tate was not the psycho who killed the kids at school, but rather his evil twin?
And that's why Tate doesn't remember doing it. Sadly, when the SWAT team arrived, Tate was the one they found. Nobody knew the brothers were twins. Maybe the bad one was usually locked in the attic with Beau?