I don't know there's any reason the Beamers which are relatively small have FTL, but I think Lexi said to Tom the reason it felt like 1G is that was how fast they were accelerating, which means if they needed to get half way then turn around to decelerate, they should have been able to reach the moon in about three and a half hours.
I had been assuming that there was no faster than light which is why the Espheni haven't brought in any reinforcements, but then the Volm started disappearing to other planets and reappearing so quickly.