RANDOM SELECTION OF BOXES

Here is what I'm trying to do:

I created a script that arranges a boxes based on a given surface. but I'm having 3 issues

1. I'm missing the boxes on the edge of the surface, and I don't now how to make them appear

2. Also I haven't been able to trim the boxes on these edges

3. I want to randomly select and group a percentage of the boxes, having at the end 2 groups of boxes (i.e. 25% and 75%).

I will appreciate if someone can help me with this.