I added two species this week to my main tank- Otocinclus and Macropodus. How will this change the dynamics? I'm not sure.
The three Otocinclus are unlikely to have any impact on the community dynamics (except, of course, for waste processing and oxygen demands, which are, of course, valid considerations). The attracted some interest initially, primarily from the Angels, but they have since faded into the background. They hang around (literally) on the sides of the tank (usually the back) or the Echinodorus leaves and stems, and don't move too much. Hopefully they will keep the algal growth in check (not that I have a lot of algae, thanks, I'm sure, to my plant population). I'm a little more concerned about keeping them fed. I suspect that they will be ok, even if the algal growth is limited - I'm guessing they can eat the plants, if nothing else. Still, I have decided to try feeding them some "parboiled" spinach. The websites I looked at suggested using "plant clips" (which apparently clip onto the side of the tank) to hold the spinach/zucchini/brussel sprouts, but I just tucked the spinach leaves into the gravel. I added the spinach this morning, and so far there has been very little interest from anyone - two of the platies are paying some attention to the leaves, but the Otocinclus have not made it anywhere near the bottom of the tank. Maybe I really do need some sort of a "plant clip", maybe the Otocinclus won't venture onto the bottom of the tank. We'll see, I suppose.
The other new additions are a very different story. Two Macropodus (hopefully a male and a female) have impacted just about every aspect of life in the tank - among other things, bumping the male fighter off his post as "top dog" in the tank. Although they have a reputation for aggression, so far I have been lucky - they have stared down the fighters and chased the platies a little, but overall nothing too serious. On the other hand, the male is harassing the female a bit much...still, there's enough cover for there to get some peace, hopefully. She doesn't look terribly happy overall though.
The presence of other anabantids have definitely affected the fighters. Initially the male got a lot paler, and the female a lot redder. Since then, he has recovered a lot of his colour, and she has lost some of hers, but their behaviour has definitely changed.