SnapTime

Check Smogon on how to make a good Rain team.

Focus Sash only works once. Choice items should be good. Check Scizor's Smogon article for more info.

Try both first then decide

Scizor is neutral to SR, but it's still a fine choice, as it resists Starmie's weaknesses and Starmie can cover its Fire weakness. Gengar is still fine as your spinblocker. Flygon isn't meant to defend; it's meant to scout and revenge kill.

Yes. Then again, Flygon is the better option because your team is very Stealth Rock-weak and Flygon resists SR. It's not frail, but it's not bulky either.

Not needed. It's too bulky. Focus Sash is better on very frail 'mons or suicide leads (Metagross is too bulky to be a suicide lead). Lum Berry cures any status Metagross takes while Occa Berry weakens supereffective Fire moves Metagross would take (only once); they're the preferred items on lead Metagross.