That doesn't work, for two reasons.
A - The Krox ship is far faster than your ships; it would probably have time to get both fleets before either reaches the planet.
More importantly,
B - You can't build a fleet - the Krox destroy the ships as you build them. And before long, they'll start capturing your planets.
As far as I can see, the only way this scenario is at all possible is if the Krox decide to pick on the Altarians for long enough that they allow you to build some sort of force. But there's no way to control that, so at best, this scenario seems to be totally up to luck - either that, or there's some trick I'm missing.
If anyone knows how to reliabely, or semi-reliably, win this, please could you post an strategy answer? Or at least very direct hints?