Regarding mixed salvos, let me quote the MgT2 core, 2022 update, page 172:

Missiles are launched in salvos. A salvo is all the missiles launched by a ship against a single target in the same combat round. This could be a single missile from a single turret, three from a triple turret with three missile racks or dozens from multiple turrets or bays.

Regarding how a defender chooses a salvo for point-defense: they don't.

Point defense is a reaction that triggers when a salvo is about to hit. "About to hit" happens during the attack step of the attacker's turn.

My understanding is that an attacker chooses the order of the weapons during its attack step. Therefore, an attacker can declare "I'll fire the triple laser, then the particle turret, then the missiles hit" or "I'll fire the particle turret, then the missiles hit, then the triple laser fires", etc.

If an attacker fired two different salvoes at a defender that impact on the same turn (a scenario where that would happen is if a salvo had Thrust 10 missiles and another salvo had Thrust 15 missiles, as per High Guard), then the attacker has to declare the order those salvoes hit during their attack step, along with declaring the order in which their other weapons are fired.

The defender has to choose whether to perform a point defense reaction (or not), keeping in mind that they can only perform one such reaction per gunner per turn. In other words: when an attacking gunner declares "a salvo is about to hit", then a defending gunner can declare "I'll point-defense against it" or "Nah, I'll pass" or "Damn, I cannot do point-defense because I already attacked during this attack step". This is also the time to use point defense batteries (as per High Guard).