Until now there is no single official source indicating that Messi or Neymar will play that game against Brest. Pochettino already indicated that he is studying whether to include Messi or not in that game. Comparing to the previous match and for the moment, only Di Maria, Verratti, Donnarumma, Marquinhos will be additionall ypresent with the team in Brest.
Brest is not a big club in Lique I so PSG can beat them without those stars. Messi just came back after a long vacation and he needs one month of training to play in real matches. PSG are powerful and they don't need Messi only for Lique I. They need a healthy Messi for Champions League.
When all of them are available for matches, PSG will have two powerful squads that are able to beat all clubs in Lique I. They will complete Lique I 10 rounds before the final days, I believe so.
The appearance of Donnarumma damages the solid position of Keylor Navas. However, having Navas as a substitute goalkeeper is great for PSG. They will have a safe goal with official or substitute goalkeeper.