If he manages to defeat Venasaur Red knows Aang is stronger than a high-level firebreathing dragon, so yeah, he'd send out everybody. Venasaur took an attack that ravaged the landscape without croaking when fighting Deoxys.
Hence my respect for Red's Pokemon might put the match more in Aang's favor. I haven't been following the manga so correct me if I'm wrong, but from what I know, if his Pokemon and an opponent have a relatively close fight, and the opponent wins, his standard response is to send out one other Pokemon to continue the battle. Only when he knows his opponent is uber powerful, or his initial pick gets absolutely thrashed does he respond with the gangbang. So if Aang were to thrash Venosaur, yes, he would send out the remaining five, but if the fight is close and Red is only exposed to Aang's fire offense, I think the likely response would be Poliwrath.
By that time, though, Aang would be smart enough to realize that Red's vocal commands are the source of the Pokemon's attacks. Personally, I would realize this seconds into the match, so Aang should be able to as well. I mean, it's obvious. Then all it takes is a gust of wind to the head and Red should be out cold like a baby.
Unless he has some superhuman durability that I'm unaware of.
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"Give him a fair trial. Then shoot him." - Napoleon Bonaparte after the capture of Andreas Hofer