Hey there :)
Its either/or.
Lets say you have a Mech with an ER PPC in the left torso and 3 Medium pulse lasers in the right.
You can have a Turret in either the left torsp, or the right, so you could put either the pulse lasers, or the PPC, depending on what torso its in. Regarding the number of weapons, you can put in as many as you want, it just drives the weight of the turret up, so the more you put in, the heavier it gets.