Most of the veneering I've done has been with commercial solid products. Their thickness is around 1/40" and will make a pretty tight radius. When making my own veneer, I'll resaw and drum sand to less than 1/8" down to less than 1/16", depending on the project. I've used a lot of bending ply, but have also used poplar for the filler if I'm not making a really tight radius. I'm thinking a 6' radius could be done with any type of wood, if you process it to 1/8" thick or less.