Any fish can end up being a shrimp killer, but I've had great luck with guppies. I've kept tons of them and never had a problem mixing them with shrimp. The mollies are more of a risk, but still they may be fine. You may also end up wanting to add other fish that may be a risk to shrimp, but you could always just go with some ghost shrimp. You can usually get atleast 3/$1 so if some get plucked off it's no biggie. Shrimp have very little bio-load, so you can easily add a bunch of them right along with the cories if you want to. For the cories, I do supplement with a high quality sinking wafer every other day or so. If you aren't overfeeding, there shouldn't be too much going to the bottom.