The only time I would ever use regular Gaara is if I was a wind main during Nine Tails. As for farming fragments, you should be farming fragments as much as you can to get all your ninja to 5*.

I wouldn't worry much about line ups until you're 70+. Save up seal scrolls and draw from Kage treasure. Sage Naruto is good, but I would prioritize Ao over Sage Naruto. You can get Ao as a regular ninja drop from Kage treasure.