From what we have been hearing about Selby, the Wall comparisons don't sound too far off. His outside shot may be even a little better than Wall. I think it might take Selby two years to match to get to Wall's level, but I could certainly see him getting there.
As for Harris, he and Cousins are completely different players. Harris has much more of an outside game and can take bigger guys to the hole and post up smaller guys. Cousins is just an absolute beast inside and a much better rebounder and defender.