Both of their salaries work. Both teams would benefit on this trade. Toronto desperately needs a starting PF. Ross still has potential and maybe Brad could work his magic on him.
Toronto has Patrick Patterson, Luis Scola, James Johnson, and now Anthony Bennett at PF. They are not desperate for another.
IMO none of those power forwards you listed are starting caliber. Sullinger is arguably much better in terms of skill and potential.
That depends on what role you want your starting PF to play. Sullinger may be the best talent of the bunch, but in terms of a stretch 4 (the best option to play alongside Valanciunas), Patterson is better than Sullinger (or at least, has a demonstrably better track record as a 3 point shooter).
A platoon of Patterson (shooting)/Johnson (defense)/Scola (scoring/smallball center) is plenty effective for Toronto. Why would they trade away depth at the 2 (leaving Powell the only backup to DeRozan) for small (if any) gains to the starting lineup?