The "inch per gallon" so-called "rule" has other problems too. It is obviously only referencing the ratio of body mass to water volume. There are several other even more important factors to consider when it comes to the number of fish in a given tank.
We have already touched on a couple of these, but I will give the list so it is complete. The water parameters, aquascape, filter flow, live plants, and fish species all impact the stocking. The last item, fish species, has several components: the number needed for the species (shoaling fish need certain numbers), the size of the individual fish, the temperament, their behaviours (active swimmers, sedate fish), the water parameter preferences, and environmental preferences--does this particular fish need wood, or rock, or sand, or plants, or water current/no water current, or dim light, or...--and so forth. This is why the online calculators do not always meet the mark; the specific factors are difficult if not impossible to programme into such a calculator; it takes human thinking.
As a simple example from my own fish room, one of my tanks is a 29g, and it has 63 fish, whose total adult lengths would be about 102 inches. Yet this aquarium is not overstocked by any degree, simply because of the total compatibility of all species (regarding water parameters, environment, activity level, plus temperament). But if I were to stock this same tank with say Tiger Barbs, I would have 12-15 of only this one species and that would be it for this tank, because of their specific traits.
Turning to your list, boshk. I would leave out the guppies, primarily due to differing water parameters, but also this is not the healthiest of fish these days. Similarly for the dwarf gourami (
Trichogaster lalius), this is a risk. The Honey Gourami (
Trichogaster chuna) would be a better choice if you want a similarly-sized gourami.
The salt and pepper cory...is this
Corydoras habrosus, or
Corydoras paleatus? The latter is often commonly called the pepper cory, the former the salt and pepper (in NA anyway), but they are two very different corys with differing needs. Scientific names are always more reliable so everyone knows exactly which species is being mentioned.
On the algae/otocinclus question...otocinclus should only be introduced to an established (mature) aquarium, with a reasonable biofilm on the surfaces where algae will grow. This fish is wild caught, and often nearly-starved when purchased, so it needs a "natural" tank or it may completely starve. It will learn to accept prepared foods like sinking algae foods, but not always at first. Also, it only eats common green or brown (diatoms) algae, it will not touch the more problematic algae, just so you know.
Byron.