Amoung S60, i think N71 inclusion will be helpful, becoz it is a lead device in java verified testing.
I like to suggest below criteria for device selection...
1) All lead devices in each subset of models, which have a common platform can be included. Eg, pls see Java verified supported models pdf.
2) in my opinion, lead devices can be the lowest device in that group with respect to heap memory and processor power, while it will retain all other features of its group.
3) display size is a important criteria for this selection process and it is good to included devices of all display size in each platform. But, i personally dont need testing on devices with display less than 176x220.