you can breed rosys by getting the largest you can find( at least 3 inches). Then stick them in 22c water water and then raise the temp to induce spawning.
yup
if Os don't get good nutrition they tend to get HITH
so as a treat the rosies would be nice and fun to watch but do get some variety in food worms would do great
and shrimp as you have already mentioned are also great for nutritional values