Use also random for all you want!
I generally randomize my groups: nbr of units, type of units, even skills (in a slot).
Prepare your group in an array: for "_i" from 1 to whatYouwant do {[] pushback (type of unit randomized)}
then this array in spawnGroup
Same for waypoints: an array, then pick them with arrays_of_waypoints select floor random (count array_of_waypoints).
(a new dev command: selectRandom , will be issued next stable version).

But the main challenge, when filling a town, is that you dramatically drop down FPS with all these units/objects!
So, have on mind a way to spawn /de-spawn units along with player(s) distance(s).