Melo's game has someone go down in the last 3 years and I would say that he is already past his prime. But he can still hit those outside shots that's why he was given a second chance by the Lakers base on his performance with the Blazers.
For Howard, he already lost his spot as the center position and has been playing on different teams. If he will be the starting center for the Lakers, then maybe he can increased his ppg and rebounds and that will be big enough contribution coming from him. And we have seen how he blend with the Lebron type of game, so again, he might have a good output as a Laker.
I don't think Melo will be in the starting line up.
And LeBron might play much at the PF position, while AD might do a little bit of playing the Center position.
However my current prediction with the Lakers starting five would be;
Pg : Schroeder
Sg : Westbrook
Sf : LeBron
Pf : Davis
C : Howard
If the Lakers decide to have a quick and aggressive starting line up, they could put AD at the Center and LeBron at the PF and add Ellington at the SF position.
Melo would be a good depth for the second unit for the Lakers.
So, these are variables that can be analyzed.
I have no doubt that Melo or Howard can start as a starter in some games. We see for example what DRose has shown in some games in the past.
LeBron is a wild player (I think he is the most flexible and well-rounded player in NBA history). He can easily play as SG, SF and PF. (maybe PG or C)
The Lakers will have several options to mix the team, with the goal of building a team that has great chemistry.
If Melo and Howard only play half what they were in their prime, I'm pretty sure the Lakers will be champions this season! But it's too early for me to say this accurately
But I still believe they will collaborate much more with their experience than their Lakers skills.