predators select prey size carefully. There's something called optimal foraging strategy that means animals will get food in the most profitable way, i.e., the most energy gained from prey compared to the most energy used up getting that prey. This applies equally to herbivores as well as carnivores.
Work done several decades ago on sunfish demonstrated that they will swim a small distance to catch small daphnia but a longer distance to catch bigger daphnia. Small daphnia at a far distance will be ignored if small daphnia closer by are present. Since small daphnia provide little energy, the sunfish will only expend a small amount of energy catching them. Bigger daphnia return more energy, so it's worthwhile swimming more to catch them. If less food is about, the sunfish becomes less picky, swimming further to take smaller prey. When food is abundant, the sunfish is more picky, and will swim shorter distances for smaller prey.
I'd caution against using optimal foraging theory as an excuse to mix small fish with big predators though. While I've seen it done with, for example, piranhas in big tanks with guppies, it isn't 100% reliable. In an aquarium fish will adopt learned behaviours, connecting the presence of humans with the appearance of food. That's probably going to override their optimal foraging programming.
