I like Sullinger. In some ways he's like a poor man's Boogie. In that way he wouldn't be a terrible fit playing behind Cousins and getting 10-12 minutes per game. But with Papagiannis and (for now) Koufos, there wasn't a need for a backup C.
As a PF I don't think Sullinger would be a fit at all. He's got a decent shot but he's still very inefficient offensively, can't guard quicker fours, isn't great defensively overall and in general his game doesn't mesh with Cousins. Tolliver is a much better fit in that regard.
On the surface Sullinger shot 43% from the field and Tolliver shot 38.5% but the reality is that Tolliver shot a higher percentage on both 2 point shots (50% vs 46%) and 3 point shots (36% vs 28%). The difference is that 82% of Tolliver's shots were from behind the arc and only 13% of Sullingers were. If you're building around Cousins which guy looks like a better fit?
Tolliver is also a better defender. So while it is still likely an overpay (1) that's just a fact of life for the Sacramento Kings and (2) they are getting a guy who is a better fit next to the team's best player.