The broader question: is a filter rated at tank size, or less, good enough? Depends on many things, starting with what it is that you want the filter to do.
For mechanical filtration, which is trapping debris for easy removal and, perhaps, the most critical aspect in a planted tank, then you can judge based upon what you see accumulating every week. My filter turnover is 5-6 times / hour, which is enough to trap quite a bit and keeps the tank free of most debris. This, in turn, minimizes organic build-up. This may be the most important part of a filtersí purpose. Weekly cleaning of the filter is enough. If it isnít, then you need to look at the health of your plants and feeding regimen.
The purpose of biological filtration is to keep the ammonia down. The substrate and other surfaces will also do this, but much more slowly, as will healthy plants. If you have a sufficient quantity of healthy plants, you may not need a bio-filter at all. I have a 29-gal heavily planted, high tech tank (rapidly growing plants consume a lot of ammonia) with a very heavy fish load (2.3 times what is recommended on AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor
). I have only mechanical filtration in my filter. All ammonia created by the fish is consumed by the plants and BB in the substrate and on the surfaces.
If I had a low tech tank (Iím assuming yours is) with the small fish load that you have, I would also try a filter with only mechanical filtration (you should gradually reduce the bio-filter if trying this). I believe, as many do, that plants prefer the ammonia/ammonium form of nitrogen to the nitrates created by bio-media in a filter. Why not let them have first access to it? If your pH is below 7.0 all the better because then the ammonia is in the relatively harmless ammonium form.
Having good circulation is as important as having good mechanical filtration. You may need a pump to boost circulation if your filter wonít do it well enough. I have two of the smallest Hydor Koraliaís pointing straight up to not only give me good surface agitation, but also to provide good circulation throughout the tank where dead spot otherwise would develop.
In sum; Iíd focus on making sure the filter can remove all the debris without clogging over a weeksí period of time. Bio-media is less important, and can be monitored for need, based upon ammonia readings.