I personally would not have a 4-inch pleco in so small a tank, but more for the visual than anything else. Pleco are different behaviour fish again, and if the water is well maintained it would (or should) not have health issues. But it may look, or it would to me, out of place in a relatively small space.
I'm hoping you can return the leaf fish, that would be wise and best for the fish, hoping someone who understands its needs can acquire it. But we can't stop stores from selling inappropriate fish.
I like to see small fish in "small" tanks and usually recommend such because having small fish means you can have more fish, and that means more interest in the aquarium, which will be more enjoyable and less likely to cause one to give up out of despair. There are many "dwarf" type species available, though sometimes one has to look for them. Many of these will be wild caught, which means paying close attention to their needs in water parameters (hardness, pH, temp).
Those fish profiles we have include many of the dwarf types, under the Cyprinids and Characins particularly, but there are also Endlers in the livebearer group that are not yet in the profiles. If you can tell us your tap water parameters (hardness and pH) this would allow us to suggest some options.