The majority of the canon is from the past Doctor book Divided Loyalties. The middle part of that book is an extended flashback to the Deca, and the rest of it goes into them a bit too. Although fanon does ignore a few bits of Divided Loyalties, even though that’s where the main idea comes from. And reinterpret bits of it too (we in the Vansell fandom will spend a long time explaining how he’s a precious baby trying to do the right thing and Theta was a jerk to him).
As for the rest of canon, most of the Deca appeared in other things first, but just as Time Lords the Doctor knew; Divided Loyalties is the first (and only??) thing to mention the Deca. All the other information tends to be scattered around.
The Monk (Mortimus) first appears in The Time Meddler and it’s implied that he and the Doctor had never met before. He gets the name Mortimus in one of the novels, I think (possibly a comic? But not on the TV show).
The War Chief (Magnus) first appears in The War Games. He and the Doctor mention that they knew each other before, but don’t go into it. There’s information about his backstory and the reason for his feud with Theta in the comic Flashback, and again he gets the name Magnus in the EU.
Drax is from The Armageddon Factor and, other than the Master, was the first Time Lord to say on screen that they’d been at the Academy with the Doctor. Drax is the one who tells us they were in the Class of 92 together and the first person to use the nickname Theta Sigma.
The rest of the Deca don’t appear in the TV show. Vansell first appears in the Big Finish audio The Sirens of Time and doesn’t really interact with the Doctor at all in it. (Which makes me think that The Apocalypse Element, in which he has a much bigger role and it’s established that he knows the Doctor, was written before Divided Loyalties, even though Divided Loyalties came out first.) The Big Finish audio Neverland also mentions Vansell and the Doctor’s time in the Academy a bit.
The other three members of the Deca, Rallon, Millennia and Jelpax only appear in Divided Loyalties. (It was implied that Jelpax was the Time Lord at the start of Genesis of the Daleks, but this was retconned later.) The Celestial Toymaker goes into the Doctor’s first visit to the toyroom though, which Rallon and Millennia accompany him on in Divided Loyalties.
And I thiiink that’s basically it. Hope this helps!
EDIT: Also it’s generally accepted fanon that several members of the Deca had a crush on Theta.