I dunno much abiut barbs and their bio load, but first thing, ditch the carbon. I would set up 2 strictly for bio and 2 for mechanical. Or 3 bio and 1 mechanical. I've never owned an xp2 but the 3 and 4 suck for mechanical filtration
Word of advice, sell the renas and build a sump!
I like the tank and barbs!