I've had several pairs of size 52 Rotas taken in to an approximate 50 waist (for the same pocket flair reason). My tailor either removes one belt loop and relocates another, or relocates both depending. I haven't had an issue with the back pockets ending up too close together, but you should ask your tailor what he thinks will happen to them once he's taken in the waist.
I have this photo of Dugdale Worsted and Cashmere saved that looks pretty nice. It might be 1313 from the Super 120's and Cashmere book but I'm not sure. If so, it's 10 oz.
@venividivicibj Was the Dormeuil Ice one of these?
Probably not with the derby or monk, since they'd be more adjustable across the instep. I have low insteps, with left lower than right. Robert is great for both feet. For Rain in the same size my experience with an oxford is that the right is OK (could be a bit more snug), but I use a tongue pad on the left; with a derby, the instep for both is fine and I just tighten the left a bit more.