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.
It is usually not about which team is better when it comes to football, better teams will win most of the time and that's just how it goes, but even the team that is at the bottom could end up beating the team at the top time to time. For example, Liverpool 2 seasons ago had an unbeaten streak for a long time, maybe 30 games? Something like that, you know who was the first team that managed to beat Liverpool that season? Watford, the team that is back on premier league now but relegated that same exact season.
So, a team that relegated gave Liverpool the first loss after 30+ games, THAT does happen in football. However I do not see PSG losing this game, this is not one of those games, PSG has a big fire in their gut and they really want to beat everyone they face and with that type of fire I do not think that there are many teams in the whole world that could beat PSG.