Are the Aventine adventures considered part of DS9?
There's no formal classification, but I see the Aventine
more as a free-ranging "guest star" ship that generally appears in combination with ships and characters from other series. It was in Destiny
along with the TNG and TTN casts, Zero Sum Game
with DS9 characters, and Brinkmanship
with the TNG cast. Even A Singular Destiny
can be considered a crossover with the Bacco administration and Corps of Engineers
, among other guests. There's never actually been an "Aventine
novel" per se.
I guess you could say it's the ship/crew equivalent of a character like Gul Evek, who appeared in Maquis-related stories on TNG, DS9, and VGR, or Q, who appeared at least once on all three series and was recurring on two of them. Not series-specific, but a common element of the shared universe.