The worst you'll prob find in Xprison missions are megapol enforcers and the huge monkey guys with shields and batons, those are pretty tanky, but can be taken out easily using tech blades and other stuff you'll likely have by that point.
And your gals do get some decently lethal weapons early on. You can take out even mercs using the humble assault cannon, though it'll prob take more than a single shot. And don't neglect melee, as you can take out enemies in power armor simply by having a gal swing an axe or a hammer to their backside a few times. You can also get the krazy hannah contact going before you get a workshop, too, meaning you get access to RPG's, mortars, light, medium and heavy machineguns, all which are pretty damn deadly against anything short of maybe the star gods themselves.
And yea, if you're careful enough, you can have nearly all your gals be castaways, greatly lowering the overall upkeep. It just that I didn't really pay attention to the spike in maintenance due to using lots of peasants, since I didn't really use them much until now.