Accepting that the Guardians are dead and the Roaches will never grasp the fiscal benefits of reciprocal commerce were the Guardians part of a wider Galactic community? Do their outlying locations indicate where other groups might be found?
The lore extracted by Ram Tah clearly indicates the Thargoids were the only sentient species ever encountered by the Guardians.
Which makes sense, if one accepts the postulate that the Thargoids are the answer to the Fermi Paradox. Under this theory, there are, at most, only ever two starfaring species in the galaxy at any one time: the Thargoids, and whichever species the Thargoids are attempting to exterminate next.
You have a log reference for this? I can find no such statement in the logs.
To the contrary the Guardians are noted as being 'skilled linguists' despite only having a single language. That would indicate a need to develop translation skills.