Since my 3 shadow priests hit 61 and I am forced to level them somehow in the Outlands,
As you can't just grind instances yourself, I'm afraid there's no magic formula for this.

Do the collection quests with only three characters. It'll help alot. If you're confident in your abilities, try to get a group together for instances. Unfortunately, SPs aren't the most in-demand class for most Outland instances (where humanoid CC is worth more than undead CC), but it's worth a go.

You mentioned RaF. My approach is to take it easy from 60-70 with the teams I have in that range, because I'm not planning to do much of anything until WotLK anyway. I won't be BG/rep grinding, for example.

So I'll be maximising my RaF bonus by simply stopping completely at 60 and creating all the weird teams I ever wanted to create before my 90 day RaF bonus runs out, then I'll take stock of where I am and start pushing whichever teams I feel like pushing to 70