Six, I think... not that there's any reason to ever have installed Armada on disk, since it came out on CD at the same time
The attacks on Earth mentioned in Victory Streak are from the novel Fleet Action... they're also mentioned in Armada's Voices of War manual.
Anyway, universe-wise Armada's kind of odd...
It has 11 set sectors of space, which are nice because they help fill up the empty space needed to flesh out Confed and Kilrah... (my kingdom for a map of the Hari <G>).
The story between the Shiraak and the Lexington in the gauntlet is rather good, and we can probably assume that that's what happened to Lexington.
(Lexington is referenced in Victory Streak, BTW, in the timeline).
As for the ships, I believe the speed is a tradeoff that corresponds with how light they are...