Not in this case. You don't control that big a
chunk of space that close to Terra without some mention. The Xindi were too big and too close to disappear.
The Xindi weren't big, and they didn't control any kind of empire - they were pretty much on par with Earth of the 2150's, having a couple of colony worlds and trading with their neighbours. They were nomads without a homeworld, and barely able to work together long enough to pose a threat to Earth. Random pawns turned against Earth as part of a war through time.
Earth went on to found the Federation and become a big deal. The Xindi didn't. In a future timeline Archer was shown, the Xindi were part of the Federation in the 26th century.
Never saw that - not having tv, I stopped making the effort to catch the show in the 2nd season
The Axanar were in one of the early first season episodes - they were the aliens on the derelict ship, strung up in the ceiling. It freaked Hoshi out. A few other "name only" TOS aliens were seen as well.
Continuity issues and retcons aside, you can't call Enterprise a reboot without ignoring the Mirror Universe two parter (which crossed over TOS and ENT) or the (admittedly awful) final episode, played out on the Enterprise-D's holodeck just prior to and between early scenes of "The Pegasus". Whatever Braga said early on, he either changed his mind, was overruled or was bullshitting (like his more recent outlandish claim that Future Guy was Archer)