This is a new tank? Please dont just jump in with the mandarin. Mandarin is going to starve or make you go broke trying to keep it fed in a brand new tank. Even if they take to frozen food, they require constant grazing thru the day in an established tank that has a large population of pods. (Smaller the tank the more lush and packed it needs to be). They will eat thousands in a day. Even if you feed frozen it doesnt work well as they will peck a few bites, but that's it.
Mandarins are like a few other fish in that they don't really have stomaches, just a straight run to the intestine. In other words they have to graze all day, they can't be like my other fish where they get a big load of food stored in their stomach in the morning and that's it except for some light grazing in the tank.
Mandarins are great for adding later because just about all fish will leave them completely alone so you don't have to worry about them being harassed usually. So by all means get one, they are amazing. But really patient and wait minimum 6 months after tank established if not a year. And if starting with dry rock make sure you are getting macro algae from people and maybe some seed rock or otherwise do indeed get some various species of pods/amphipods dumped into display and sump to start getting established.
Grass shrimp won't be bothered by mandarin, nor will the cardinals eat them btw.