Just create a a unit-type array in the Trigger editor.
Set UnitType[0] = Peasant
Set UnitType[1] = Footman
Set UnitType[2] = Troll Headhunter
Set UnitType[3] = Ogre
Set UnitType[4] = Archer
......
then when you want a random unit just "Create 1 unitType[random integer between 0 and 4] at tempPoint1.
This way you also avoid creating unwanted units like heroes, buildings, critters, neutral buildings, gold mines, etc.