I would be more concerned about keeping the water clean with a good filter first.
The copper that will kill INVERTS is the amount you would find in the "Cu" based MEDICATION.
The trace amounts of copper found in HIGH quality dry foods in addition to fresh veggies are negligible.
To give you an idea.....
You would have to dump the entire container of food into the tank before the amount of copper in the food itself would harm them. The ammonia created from the decaying uneaten food will get them FIRST.
All shrimps will receive small amounts of "Cu" through their foods & require the element to grow/molt. As long as you keep up with the water changes/filtration & don't OVERFEED, the shrimps will multiply on their own.
NOTE: I'm referring to c shrimp.
Prepared foods = Spinach, romaine, zucchini, summer squash, pumpkin, carrots, stinging nettles, IAL, oak, dandelions, etc.... are other items that people have fed their shrimp. If something is too fresh & the cellular structure of the food provided isn't slightly decomposed, the shrimp will often ignore the item.
The frozen foods you have in mind are probably the riskiest to feed when you're starting off. I would advise against using those.
Protein based foods will mess up your water the quickest.
Stick with plant & pellet based in addition to the leafy greens you find in your salad.