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Rock is my least favorite type and three out of four of the rock type gym leaders are the first gym so that's what I'd be.
Personally I headcanon that gym leaders scale their team depending on the badges of the trainers they are versing, there is no other reason why Brock would officially qualify as a gym leader alongside Giovanni and Koga. So with that mentality this would be my full team and if complete noob trainers came I'd only use Sudowoodo and Shuckle.