Yes, all fish in the tank will be fed at once when you drop the food into the water. If any fish in the tank aren't "hungry" they will lose a small amount of health from overfeeding, but it's usually not a problem if they aren't already losing health from sickness or inadequate Fish Environment. If they're otherwise healthy, they will quickly recover from the overfeeding.

Each tank is completely separate from the other tanks when it comes to food and chemicals. Food dropped into a tank only feeds the fish in that tank, so if you have fish that you don't want to feed (or subject to the chemicals) you can move them to a different tank before dumping stuff into the water!