SC2Mapster

Have a look at this map.

http://sc2.grum.nl/randomspawn.SC2Map
(This is a different and updated version of the map I previously linked)

Places a random unit at a random location once every second.  Simple, clean, and efficient.  Easy to read and understand.

No offense, but I just don't see the merits of your system over the one in this map.  (Thanks Grum for the map)

Also, for randomizing the units:

Alright make a Variable called "RandomUnits" as a Unit Type and set the array to 4
Run an On Initialization Trigger that sets each one (1-4) on that trigger to a type of unit
Then on a the trigger that spawns the units for a player missing units add a local variable (call it X, an integer) and then have the trigger set X equal to a random number between 1 and 4
Have it Create Unit of Type (Type being RandomUnits(X))
Vula

Or

What I did was
Variable with array size of 4
monsterp = (Any units in Region 001 owned by player Any Player matching Excluded: Missile, Dead, Hidden, with at most Any Amount) <Unit Group[4]>

Trigger action
Untitled Trigger 001
Events
Timer - Every 1.0 seconds of Game Time
Local Variables
Conditions
Actions
Unit - Create 1 (Unit type of (Random Living unit from monsterp)) for player 1 at (Random point in Region 002) using default facing (No Options)

EDIT: My solution does have the need to have the units on the map, but that's not a big deal. Lump all my units into one corner of the map, have the trigger reveal them all(apparently the game can't pick from non revealed units) pick one at random and spawn it, and then hide them all again. The players will be none the wiser.

There has to be a simpler way to randomize than this...

I look at your map, then I look at

http://sc2.grum.nl/randomspawn.SC2Map

Although your map also creates a random unit, surely you can simplify your system.