I have a set of 4 gunmetal grey 15x7 4 inch bs diamond racing pros on the car right now but i also have a set of black riken mesh wheel knock offs, 15x7 rear 4 inch bs and 15x6 front with 4.5 inch backspace.

both wheels need tires at this point trying to decide which one to get tires for.

 

It's so easy, economical and practical to get tires for 15x7's. Exactly why I just bought my set as you already know.
So,
That's how I'm going to answer that question.
Besides, I don't know why you'd want to run a staggered set like your rikens. The only reason why I would run a staggered set is if maybe i wanted to change my mechanical grip front/rear by playing with wheel placement and tire size.

Regardless, Diamonds look MFin' Badass! So, keep em' and rubber em' up, I mean mount tires on them.