My opinion is that they are not the same type of creature, not at all. First, It arrived from "outside," i.e. outside of everything. It is a monster whose true essence is walled off from our various realities. Pieces of the thing sometimes get in through pinholes, as is my opinion of Pennywise. The Tommyknockers are an alien species grounded very much in self-serving science. Their life cycle appears to be entirely within the confines of our reality. That doesn't make them any less horrific, but they don't seem to cast a metaphysical shadow. The creatures from Under the Dome are vastly powerful creatures, but I still don't think they are directly related to Pennywise. They are largely oblivious to our existence, we are ants to them at best, make-believe at worst. The thing beyond all the realities is aware of its victims on a human scale as well as a cosmic one. It is aware of what it does and the different scales at which it interacts. I suspect the symbols which seem to be in common in the various tales have more to do with telepathic contact. Powerful, telepathic entities often speak directly into the minds of others, or perhaps the weaker or non-telepathic entities simply perceive things on a level they can understand. The symbol on the door into It's final lair was simply fear, and appeared different (as the monster often did) to the Losers at one point. Do you see what I mean? I expect those "symbols" at various places are merely telepathic warnings or statements which lesser minds simply visualize as humans do, in symbols and glyph.
Well, what I was saying was that we know It landed on earth from space from the smoke hole incident in the book. And it seems to have happened millions of years ago, maybe before the dinosaurs. So I though maybe the leatherheads or the Tommyknockers are what It's species evolved into after millions of years.