So your plans are for saltwater mollies. Whether you breed them in salt vs fresh it's the same, other than it costing more time and money to do salt. Check out some freshwater forums for specifics on breeding mollies, it's actually pretty easy, but there are some options to consider. I no longer breed any of my feeders, the time involved wasn't for me. Growing them up to the size I needed is what stopped me years ago, most of my guys are pretty beefy now.
I would check the freshwater forum on the cherry shrimp question as well. Size and acclimation would be the reason I would opt no for the cherry shrimp. While you may feed one or two babies, one of my beast would put down at least 50 in a weeks time, that's alot of breeding. You can drop some ghosties in a sw tank with no acclimation and they will live for days or even weeks, at least long enough for whatever is in the tank to hunt them down. Ghosties are actually a variety of shrimp that lives in fresh, brackish, and marine.
I have an lfs 2 miles away that carries ghosties and have a 2 for 1 sale on freshwater on Tuesdays. So I get mollies and fancy guppies on Tuesday. They also get alot customers bringing in babies so I can baby mollies 10/$1 sometimes and smaller sized ones 4 or 5/$1. So that became more attractive to me than breeding my own. Here's a pic of my feeder tank, ghosties and feeder guppies aren't picky; but the fancy guppies and mollies need a more stable temp and cleaner water.