For sure it can be mixed with food. Over the years I have done so. However, I keep mostly smaller fish, so I pretty much just mixed it with flake food. I would put a bunch of flake into a small bowl. I would sprinkle in metro and then as a small amount of water ans stir gently. I was trying to mix multiple dosings, I would immediately feed a dose to the tank and then freeze the rrest. O cpuld then chip off a piece and drop it into the tank where it rapidly deforsted and sopred just like normal flak feeding does.
Metro tastes lousy to fish. it is important not to feed anything besides the metro infused food when dosing metro this way.
If you search via Google you will find different suggested mixes for food. Here are two I found:
This one was re SeaChem Moetroplex and was on the AmericanAqaurium Products site:
https://www.americanaquariumproducts.com/Seachem.html#metronidazole
This one was from the Aquarium medications site. There is a lot of info re Metro there.
https://www.aquarium-medications.com/2012/07/metronidazole-by-seachem-api-more.html
I have never kept rays, so I am not at all informed on what they will eat. However, there are a few sites where one can buy metro infused food for fish, Here is one such site
https://kensfish.com/collections/medicated-food Here is another
https://www.brineshrimpdirect.com/flake-diets-flake-food/medicated-brine-shrimp-flake/
And here is something which may indicate that Metro may not be around much longer as a treatment for fish
FDA Warning Letter- Oct 2020