The planet defenses.

[Tsagoth]

I wonder what will happen if a reinforcement fleet arrives in mid combat. Will the new fleet be allowed to join?

Also, what if the planet that I attack has fleets from 2 different players in the planets orbit? Will I have to fight those fleets at well?

Yes, the reinforcements should join the fight. Spectators are not involved. If A attacks B, and C & D have ships there, they will not become part of it, unless they decide to attack A or B.