the inch per gallon lie is garbage and should be treated as such. it does not take into account the space needs, temperment, behavioral needs, waste, and basic water needs of different fish. it doesnt work for any fish, or for any person, newbie or not.
if you get four more harlequins and then one more corydoras, that will fully stock your tank.
BTW, the people at the major chain pet stores that tell you this crud contradict themselves all of the time. they tell you "one inch per gallon" and then they go and sell you a container with like 1 cup of water for a betta. bettas get to around 2.5 inches long, and thus by the "inch per gallon" lie, they would need at least a 2.5 gallon tank (but even that is to small)