The Guardians, by all accounts, were a more peaceful, tolerant society (internally) than our own, with fewer internal factions and divisions. They may have been xenophobic; we simply don't know as they were never given a chance to interact with any sentient species other than the Thargoids, who certainly are inherently xenophobic.The Guardians had "races" (local minor genetic variations causing differences in physical appearance), for example, much as Humans do, yet they never seemed to have fought inter-racial conflicts to the same extent that Humans traditionally have - this implies an inherent desire to see past superficial differences and find common ground.
I think the Guardians would have been perfectly capable of peaceful coexistence with Humans, though I think the eventual outcome of their interaction would have hinged a lot on the circumstances and nature of First Contact. Would we have been meeting as more or less technological equals, or would they have found Earth while we were still savages, and mentored/uplifted/enslaved us? If we met as "equals" (i.e. in spaceships or at space colonies, rather than one landing on the other's homeworld), would the individual representatives of each race have been curious, or apprehensive? Humans, especially the "frontier humans" who are more likely to be in First Contact scenarios, have an annoying tendency to "shoot first, then keep shooting because questions are kind of redundant by then" attitude if they feel threatened.