I think i understand what you mean, but you can't shortcut this without using the script call alot.
Step 1:We're going to need a list of the roles available to the AI and Player. For this create an array, I'll use variable ID 1.
$game_variables[1] = [0,1,2,3,4,5]
The array now holds 6 numbers (0-5) that represent the roles available.
Step 2:Now use a random number generator again, but a scripted one.
size = $game_variables[1].size
role = rand(size)
$game_variables[2] = role
$game_variables[1].delete_at(role)
So, what this will do, is create a random number from 0-5 (in this case) and assign it to variable ID 2. Then it will delete that role from the list.
Step 3:All that's left to do is assign the role that we got from the code above. Which you can do through a typical event branch.
@> Control Variables : [0003 AI_1 ROLE] = Variable [0002: ROLE ASSIGNMENT]
So to do this for all the computers, you could just copy and paste this exact process and change the assignment at the end or create a page for each one to keep it neat. The easiest way would be to loop it, but you would at least need to tell me your event IDs to do this.