adding C45 to it, you're going down a dunkleweizen path... I don't think you need it at all. I'd say 50/50 mix of pale/wheat. If you want/feel you need to darken it up a bit or add some 'sweetness' then use vienna instead of the pale malt. As for how much of rhubarb to use.. its been so long since I've had it, I remember its tart but not how strong the flavor is. I would say add it to secondary though.