I have 20" rims and I can tell you all without them that the ride is not harsh or rough. Yes, it will have less payload but that is it. If this is a truck that will be used where the extra payload is not needed and the OP likes the 20" rims then he should not let that deter his decision. Put another way - I would not want a ride any "softer" than I have with my rims and tires.
As for the 6.5' bed length - I believe that is normal since there is a longer bed than that unless I am mistaken. I would not want anything shorter than that for a full size truck. Make adjustments for the bigger vehicle that's all. You will get used to it.