Ajuga is only suitable for a small scale groundcover. It often develops crown rot and then dies off in sections. In a small area, it refills pretty fast, but I'd never use it for a large area. And a low groundcover, only a couple of inches off the ground, couldn't hide poison ivy--the PI would outgrow it in no time. I never worry about snakes either. They are more scared of you than you are of them. Anyone moving through an area would be making enough noise to scare off the snakes. I do have copperheads, but they are few and far between and in all my 70 years they've never been a problem. You probably don't have any poisonous snakes in your area and the good ones aren't worth worrying about.

And, BTW, what is the existing groundcover?