I'll usually jump in a colony frigate just before the fight is over. So even if my ally jumps in after the fight with a colony frig, my frig will reach 100 antimatter first = my planet.
I do the same thing, but she usually jumps in her frig when its still not safe enough to risk it.
AND I am still fighting pirates, I'll grab my fleet and retreat. The pirates focus on the allied colony frigate and it will jump out, at which point I return with my fleet and finish taking the planet
thing is by that time for me, there are only a few weak pirates left, and they cannot do sufficient damage to the colony to stop it (if its got enough AM)
That's what I do to, with the akkan cruiser. However, my computer has taken it a step further, they will let an enemy wipe out one of my planets, then proceed to colonize it. (as in, they'll keep jumping a ship or two every couple minutes to see if I'm dead or not, then when I am, they colonize it.
I've never lost a battle, as I'm behind a buffer of her planets.