I'll like to note that Ryoma got chosen as representative despite his "breaking the rule" behavior which is a testament to his power over other representative, and actually Seigaku got 3 select that way. If Tezuka doesn't play for German, which is overall seemingly stronger team than Japan right now (Which would also suggest his power is over other representative in Japan), so no, they didn't play for Japan. But they got even more impressive feats than playing for japan.
But really though, Seigaku power is over Rikkai as a team it's not even funny. Tezuka and Ryoma alone are worth the ENTIRE rikkai team.
Of course, I'm not saying this without evidence. Let's look at Ryoma achievement of beating up 24 representative who are tentative #14 of USA in a row. To put it in perspective that's easier to see, that's 24 players who are potentially Hakamada, who is #14 in Japan, because Japan as a country is suggest in the manga to be a pretty weak country in tennis. To see how much stronger Hakamada is stronger than Rikkai team, he's stronger than Mitsuya, who beat Renji, THE 3 DEMON, with 100% certainty. That seems to suggest that Hakadama is stronger than most people in Rikkai except Yukimura and Sanada, the stronger members of the 3 demon.
Assuming players in Rikkai is on the same level as Hakamada, which is actually pretty unlikely and view them too high, and for sake of preventing future discussion, let's just assue Niou=2Hakamada. That's 7 off the list, which is pretty easy for Ryoma to handle. Then he still have stamina to play 17 more Hakamada, so I doubt he'll have problem playing against Sanada or Yukimura. He doesn't nee to use PoP. Glowing Shot is enough to beat them. But if it's not, PoP all but ensure that Ryoma can beat them anyways.
Then Tezuka can handle the other player.. There, 2 members of Seigaku> entire Rikkai. The rest of Seigaku are simply the bonus power that Seigaku as a team have over Rikkai.
Though I agree Ryoma and Tezuka would beat him, but not with a too severe score.