One way of getting a random selection from a specific list of numbers is to put the list in an array and use random(yourArray.length) to choose the index of the item in the array (remembering that the index needs to be an int).

Strangely enough your second request is one circumstance where you could almost justify using delay(); just bear in mind that this impacts on when draw refreshes the screen and it may be problematic if you need other things to happen at a faster framerate.  The alternative and probably safer option would be to use a timer (see millis() or just search the site) to determine when the ellipse draw update code is run...

Put the alpha values in an ArrayList.
Take the value at index taken between 0 and arrayList.size()-1
Remove this value from the array list: thus you draw the next value in the smaller list, and are sure not to draw again previous values.
Repeat.

The main problem you're having is that you're trying to reference 'width' before setup has run and that's a really bad idea since IIRC it doesn't get set till after...  Anyway the answer is simple enough: You can declare the variable before setup so it's 'global' (i.e. can be referenced in draw()) and assign it a value in setup after size() has been called:

Code:

int[] als = {10,40,60,80,90,140 };
//int s = second();
// declare the variable as global
int largo;

void setup() {
  // size() should *always* come first:
  size(800,800);
  // assign the value to the variable
  largo = width/10;
  
  smooth();   
  frameRate(2);
}
 
void draw(){
  background (255);
  for (int x = largo/2; x < width; x+= width/10-largo/2){
    for (int y = largo/2; y < height; y+= width/10-largo/2)  {
      noStroke();   
      fill (0,als[ int( random (6) ) ] ) ;
      ellipse(x,y,largo,largo);
    }
  }
} 

Mind you I think you may still have problems with some values as it looks like your draw loop contains a few magic number - e.g. width/10

Replacing the 10 with a variable, both in the assignment of largo and in those width/10 references, seems to work regardless of the value used (though it gets a bit slow if you set it too high):

Code:

int[] als = {10,40,60,80,90,140 };
//int s = second();
// declare the variable as global
int largo;
int divisor = 30;

void setup() {
  // size() should *always* come first:
  size(800,800);
  // assign the value to the variable
  largo = width/divisor;
  
  smooth();   
  frameRate(2);
}
 
void draw(){
  background (255);
  for (int x = largo/2; x < width; x+= width/divisor-largo/2){
    for (int y = largo/2; y < height; y+= width/divisor-largo/2)  {
      noStroke();   
      fill (0,als[ int( random (6) ) ] ) ;
      ellipse(x,y,largo,largo);
    }
  }
}