The Brazil international doubled PSG's lead after Dani Alves' opener, but the Bretons fought back through a Presnel Kimpembe own goal and a Mexer strike before winning on penalties after Christopher Nkunku missed his kick.
Speaking in the mixed zone after the final whistle at Stade de France, Neymar urged his younger teammates to listen more to the older and more experienced members of the squad.
"We need to be more like men in the dressing room, we must be more united," he said. "I see a lot of young players who are, not lost exactly, but they lack more ear than mouth.
"They must listen more than they talk. Sometimes a more experienced guy speaks, and they answer back. The coach himself speaks, and they retort. This is not how a team goes far.
"We are more experienced, so they must respect us a bit more, listen a bit more. I had to do the same when I was starting out."
Meanwhile, Hatem Ben Arfa, who was frozen out by PSG and is currently suing the capital club for damages, was part of the winning Rennes side and wasted no time in sticking the boot in on his former employers.
"This is my greatest title," said Ben Arfa in the mixed zone. "We pulled off a Remontada, but PSG are used to that.
"It was complicated [his time with PSG]. Everything comes back to you one day, which is what I told the president [chairman and CEO Nasser Al-Khelaifi] -- the human side must be respected. When you do not do that, at some point, you pay for it.
"One day, Adrien Rabiot [currently frozen out by PSG] will come back with a different team against PSG, and they will be hurt then as well -- that is life. When you do wrong, wrong comes back on you.
"I have nothing against PSG. I love the club, since I was small -- the problem is the institution, the people in charge. Once again, the human must be respected.
"This victory brings me extraordinary joy. Regarding Nasser, this is especially important. Never underestimate your opponent -- one day, they will come back stronger."
Elsewhere, PSG coach Thomas Tuchel admitted that he needed a few days to think about evaluating his debut season, and that the situation is "complicated."
The German also admitted his surprise at Kylian Mbappe's red card in extra time but pointed out that the France international was injured pre-match.
"Kylian underwent exams at hospital after a muscle issue on Friday," said Tuchel. "That plays on your mind before a final. His red card was completely unlike him."
PSG's failure to win the Coupe de France or the Coupe de la Ligue makes this season, numerically, the least successful campaign since the Qatari takeover back in 2011 as the club have only the Ligue 1 title in hand.