I agree. 10 is very conservative, 8 is about right, even 7.
I think the 1-1 Steelix Prime line is highly recommended. As long as you make sure to never bench Onix until necessary, the Steelix will become very useful in a pinch. Especially if they manage to get all your Special Metals in the discard pile.
I also think Shaymin might be a good idea just so you can move them Special Metals around, preventing you from losing them right away to some Zekrom.
I think the 1-1 Steelix Prime line is highly recommended. As long as you make sure to never bench Onix until necessary, the Steelix will become very useful in a pinch. Especially if they manage to get all your Special Metals in the discard pile.
I also think Shaymin might be a good idea just so you can move them Special Metals around, preventing you from losing them right away to some Zekrom.