You can use sliders to change values declared globally, but not values declared indise the setup method. For instance in the code below var1 can be changed by the slider but var2 cannot.

1. float var1;
2. 
   
3. void setup(){
4.       float var2;
5.       // create ControlP5 sliders here
6. }
7. 
   
8. void draw(){
9. }

OK, for example I have this script and I want to change cluster number:

In your code cluster is an globally declared object reference and even though the actual object is created in setup it is still accessable globally. So the answer is Yes.