Fit
Waiters should only be signed if Isaac is drafted and while that is a narrow window, Isaac is a small-ball four with more upside then Dario and per reports he is favored by Brett Brown. Alongside Simmons, Waiters would give the Sixers a microwave scorer who shoots and slashes well. With Isaac on the floor the ball could move fluently in an almost Spurs-esque motion with his playmaking ability and IQ for the floor. Both guys fit well on defense as well. Waiters would guard PGs (#1 opponent field goal % among SG), Cov would handle the 2s, Simmons the 3, Isaac at the 4 if he starts due to his length and versatility he could project as a plus defender, and Embiid at the 5.
Salary
Isaac's contract here is not an issue. Waiters has expereienced a lot of past issues allowing his salary to be below max for a potential above-average guard. Waiters could be signed "cheaply" long term and would allow for extra depth signings or a star acquisition while paying for Embiid and Simmons down the road. Waiters could project around a 4 yr 54 million dollar deal based on signings of similar players such as Lance Stephenson.
Attitude and Ability
Isaac has a very positive attitude moving forward, he seems to have no character red flags and enjoys operating as a team player . Waiters brings a level of toughness and energy similar to Draymond, and while he may be a black hole at times his clutch ability and edge could push the Sixers further along. In the playoffs he shined for the Thunder and for the Heat he hit many clutch shots towards the end of the season.
Lineup
Simmons/McConnell/Bayless
Waiters/Stauskas/Korkmaz
Covington/TLC/Anderson
Saric/Isaac/Holmes
Embiid/Holmes/Okafor
Defensively Simmons would be PF/SF as previously mentioned.