With a 4-foot 55g (or whatevert gallons this is), you are in better shape than with a 3-foot as calculated. Keep in mind that these fish will (should) grow. Mollies can attain 3 inches for males, with females 5 inches and some note 6 inches. Swordtails will reach 4-5 inches in aquaria, though 6 inches is possible. Platy are smaller at maturity, at 2 inches (males) and 3 inches (females). So you can appreciate that in not that much time this tank will not look as devoid of fish as it does now when they are small. I certainly would not add more fish. With male/female swordtails (and you are good to have 1 male and 2 female to reduce stress on the individual females) you will have fry in large numbers eventually, so I hope plans are made for removing them; not all will be eaten.
I agree that cory should be in larger groups per species, but at the same time they do seem to be OK with mixed species, and you have 9 in total. I have around 50 cories in my 70g, representing some 12 species, and some of these are single specimens well over 8 years in age. It is the number in total that seems to be essential, though I will agree that five or more of each species is preferred. But here I see no cause for concern.
I'll end by commenting on your point about adding a filter, for general information. Filters and filtration is often misunderstood especially by beginning aquarists, but many seasoned aquarists also have an inaccurate idea. This is easy to understand but difficult to explain, but I'll try. Feel free to question anything.
Aquarium size and fish load has two aspects. One is the physical space needed for the fish to behave normally according to the species, and the other is the resulting water quality. Filters obviously cannot do anything for the former, they can only "help" with the latter. My use of "filter" in this last sentence refers to the aspect of filtration; filters can be essential to some fish, such as in providing more water current for certain species that require this. But generally speaking, the tank size and the individual fish are stand alone, and no filter will change this ratio as far as the physical space for the species.
It is the second aspect, the water quality, that many misunderstand. If a given aquarium has a filter suitable for the needs of that aquarium, the biological system will be about as good as it can be, all else being equal. More filters, or larger filters, do not benefit at all, unless the tank is out of balance to begin with, and in that situation, the filter is really providing an emergency function (continually) and this is not responsible fishkeeping. Take for example nitrification: the nitrifying bacteria will colonize the aquarium and be at a level to handle the available food (ammonia, etc). Adding more filters will not increase these bacteria, just the same as removing filters will not decrease them (again all else being equal). So providing the filter suitable for the tank size, and the fish load, is all you can do. Adding more fish, beyond what the system can adequately handle, will not be offset by more filters.
Byron.
so idk what to do I guess I'm just looking for suggestions maybe I could add a filter if I need? My tank is 4ft long, alittle over a foot wide, and almost 2 feet deep with a 110 Aqua clear filter and also a submersed 15 gallon filter. I have 6 baby bristle nose pleco until I am able to sex them then I have some homes for them. Accidentally put my tank alittle smaller on the aqua advisor but it is 48 inch
