Offensively, Millsap and Williams could easily coexist, and even flourish. Millsap has shown an ability to make plays from several spots on the court: The short corner, free throw line extended, top of the key out to 25+ feet, and, when needed, from the low post (where he posted one of the league's best points per possession this past season). The same can be said of Derrick Williams.
Both should also be good in transition (Millsap is a stud here, especially defensively), and Millsap has shown a knack for good positional play on both ends of the court. On account of his size, Derrick Williams is probably better suited to guard the 4 than Millsap is, so Millsap would likely be forced to guard the 3. With a poor help defender like Big Al on the court, this poses a problem. However, trotting out a frontline of Favors-Williams-Millsap would be just fine defensively, as Favors can help offset Millsap's below average (for the 3) foot speed with good help D, and would also rebound the **** out of the ball.
A Favors-Williams-Millsap frontline would be absolutely hellish for opposing teams if Favors ever harnesses his physical tools and gets mean.