Can't comment on Pikey Railbus, as I don't own one - however I can tell you that the LGB pignose is not a huge fan of R2's... it will go round them, but the wheels do rather squeal a lot! In theory though, any LGB stock should negotiate an R1 never mind an R2 (albeit with some breathtaking overhang on a 2-10-2 for example), so I would have thought you'd be OK. Your transition curve idea is a good one, I know several people who have used that trick.
Personally I'd be inclined to go for as big a radius as I dared, and disguise any sharp curves with, say, buildings or earthworks, such as a cutting or similar.
Fat Controller of the S&CGR