I was miffed by the Newton scene at the start, it seemed pointless, it was totally disconnected with the rest of the episode, and like
ruffneck23, I suspect it was a bit of a teaser for what may come up in a future episode.
You certainly can, trouble is they spent it on all the special effects, leaving no budget for additional actors during the main body of the episode.
But, they pulled it off with just the Doctor & Donna, it was gripping, exciting, a little scary, all very clever, and most enjoyable.