All you have to do to make random-looking slaves is having an alien_slave model with several body's, e.g. like the grunt's head, more versions of the body (in this case all you would change is the skin but you can also modify the model mesh a bit too).
Then, when you inert a monster_alien_slave, turn on SmartEdit and add the following key called "body", and give it a value that corresponds with the body value, e.g. the first one corresponds with 0, the second body version of the slave is 1, the third is value 2 and so on. There's a method behind this but it's unimportant for now unless you have several body-parts (like the grunts: head, body and weapon haver several options).