I'd think about the following schedule: First-wort with the Citra. Not familiar with Pioneer so I can't help you there. If it has a good flavor and you want the flavor, add the hop anywhere from 20 mins to about 5 mins, keeping in mind later gives you more flavor. The Riwaka, if it's what you want to showcase, should be late, 5 mins to 0 mins (whirlpool), perhaps as a dry hop if you want its aroma in addition to the flavor....
Short on answers, I know, but hopefully some ideas....