No, size isn't really what makes or breaks the deal. It's maturity. At 1 week, they are far too active to tolerate being under a broody that hasn't left the nest. After the chicks hatch, (or newly hatched chicks are placed under a broody) the broody takes anywhere from a couple of hours to a...
Just to clear a few things up. You're thinking oh giving her the serama chicks now, not in a week, right? If so, yes, she might accept them. Unfortunately you won't know for sure until you try it. Size isn't likely to be the reason she rejects them
It's really up to you. I'd say let Dolly continue brooding on her eggs. She's a first timer, so best let her have a smooth transition into it. If her eggs aren't viable, then serama chicks it is!
It depends on what you'd prefer. You can let Dolly raise her chicks up with the rest of the birds around, or you can separate them. If any of the birds start being overly aggressive, you should separate. A few picks here and there are to be expected, IME