I'd love to see Saul Tigh as a young toaster. Who should play the role?
No one, because there's no young Saul Tigh, the Final Five arrived at the end of the first Cylon War and the Tighs weren't younger then than they were in BSG. After Cavil killed them, Tigh was boxed for 20 years and when he reappeared in the colonies, he already looked like a slightly younger Michael Hogan with more hair. Tighs memories of fighting in the war were faked.
But I would love to see Final Five actually appearing and ending the war in the pilot with most of the movie (or mini-series?) being a flashback. I'm not sure it should be a regular series, there's always the chance it's getting cancelled without completing the story. A six or eight hours mini-series is more than enough to tell a great story, wrap up Caprica if necessary and tie it into BSG by showing the Final Five.