Most will say you are already overstocked, especially once the fish get to max size. Plecos giving off a lot more bioload than other fish (poop machines).
BUT, I always overstock (fish guy myself, not enough room for more tanks) my tanks (pretty heavily as a matter of fact), and I never have any problems in my tanks (not cramped, no aggression, no out-of-ordinary territorial disputes, no polluted water, no health problems/disease). And my fish are perfectly happy, no signs of stress and even have fish spawning in the tank regularly without doing anything special to trigger this. But I do overfilter quite a lot with plenty of biomedia to more than handle the bioloads. And I can and have gone weeks, even a month (I could go longer if I wanted to) between water changes and all water params have checked out fine and fish just as healthy.
Just my personal experience. And from my experience and with your aforementioned plant density, overfiltration and proper maintenance, I would say you are fine adding more fish to the tank. Again, most will say you are overstocked as is, but I know it's more than do able.
Although one caution that must be noted, is that since the tank is overstocked it is more easy for things to go haywire faster, for instance in a power outage or say a filter dies, since the bioload is high and livestock amount using up oxygen as well, things can go bad faster than having the same tank with less amount of fish. It still can be safely done though if you do realize this and have back up plans or are prepared for the worst.
What are your plans for additions?