Just read the ingredients .... most "prepared" marine foods contain the same basic ingredients as freshwater "prepared" foods.Anything as far as "saltwater sourced" proteins (squid,mussels,scallops,various fish) aren't going to hurt the guppies or shrimp.Just check for added trace minerals that might affect your water chemistry (if in doubt google the ingredient... or just don't use it).
One thing to check is the sodium content (yes they add salt to "marine" foods).If too much is left uneaten,and not cleaned up,it can affect salinity.... not much problem with guppies,not so good for sensitive types of freshwater shrimp.
For the record,I use "marine" foods in the variety of mixes I feed my cichlid fry without problem.Really helps their growth.
