Possibility to choose flotilla to attack

Right now I'm trying to eliminate Vodyani. Their fleets are orbiting few planets: every time it's one Arc + few Wheels.

1. if I try to attack them nearby planet X, I have 100% chance to attack exploration ships, wasting my fleet power;
2. if I try to attack them nearby planet Y (with the same fleet), I have 100% chance to attack an Arc, condemning my fleet to death.

I don't see why it so. If there are some rules that restrict to attack specific fleet (planet type, anomaly, protection order?), I want to know about them. But if there aren't such rules, I want to attack specific fleet from the list, as it goes when I press Attack, and there are fleets of 2 different factions in range.