Unfortunately it seems that all settlements get attacked at the same time. I wasn't attacked till lvl 30 and had to let my security be in the red so to speak before I could a get a proper attack. Read my forum on "Settlements" in general discussions for more.
PC version and console commands for spawning hostile enemies.
The system is weighted the more deference you have the less that settlement will be attacked
What Slaydo said. I'm level 58 and had never seen an attack other than a 3 feral ghouls attack. That was all and I had 13 settlements. So the next settlement I really didn't bother with defense-wise other than to put a cheap turret down. That settlement is now being attacked almost non-stop from super mutants.
Without console commands, don't expect a huge force to crash against your 100+ defended settlements. You'll have to create your own enemy army. Bummer.
Just put out enough water purifiers so that you have more water + food than defence.
Oooo... good thought. Will do that when I get home from work.