Given the nature of the people being sent, what is the likelihood that they will build a return gate, and if they do, what is the likelihood of them sending warships through it to seek revenge on the Imperium?
Also, how many females are you including in this sending? To allow for deaths during pregnancy and child-bearing, which do occur even now, you will need a slight preponderance of females over males. You will also need to send enough to ensure genetic diversity.
You send a warfleet through the gate and each individual ship appears in separate random locations in the targeted subsector. Most of the places these ships appear would be in interstellar space and they would need to make an additional jump from the place where they appeared to the nearest star system where it can refuel, it will expend the jump fuel to make jump-1 just to use the gate. Some ships will appear in places that are too far away to make a jump from to the nearest star system, and there is a 20% chance of a misjudged, some ships will appear in intergalactic space, missing the targeted galaxy entirely - those ships are never heard from again! Not exactly the best circumstances under which to launch an invasion.