Yikes that is quite overstocked. Are some still babies, or are they all adults?
With 12 platies I would be gravel vaccing once or twice a week atleast.
You don't really have enough room to be adding any more fish. Ottos are very sensitive to water quality, so are not suitable for an overstocked tank. Plecos are big waste producers (even the smaller ones) so wouldn't be suitable either. An aglea scraper is really your only solution. I find the aglea magnets work really well, because you don't even need to get your hands wet!