I've done some things like this, one option is to have sprites that the minions aim for, then on reaching them, aim for the base. (or 're randomise for another sprite with a higher var)
on created make a random number, for a minnion with 1: go to sprite 1, if 2, aim for sprite 2 (or give the sprite a corrisponing variable, and minnion on created, pick sprite where variable=minion.variable: find path)
In effect this makes them choose a random path.
Trouble is, if you have blocks that can block paths, they will head to the sprite reguardless
Would work well on random maps, as you could place the route sprites.
Love TD! Good luck dude