I wouldn't doubt for a second that there are other civilizations in the Milky Way that are capable of interplanetary or interstellar travel. The Covenant would have been far more likely to encounter these people in their corner of the Orion Arm, a seemingly more diverse and tightly packed population center. Through these interactions they would have undoubtedly built up their war machine to contend with these threats. By seeing the need to thus extend their influence and presence in these areas, the Covenant would have maintained the size of their fleet and later expand it as the hegemony grows to encompass new member species and ward off any future threat.