I would wait on the pleco, they don't stay small for long, plus you have to let your bio-filter do its thing. plecos are poop factorys too.
a little on cycling, first you pedal your bike...., wait wrong one.
ammonia<nitrites<nitrates, there will be a peak or spike first in ammonia, then the bacteria will catch up and this produces a peak in nitrites, another bacteria then consumes the nitrites which in turn produces nitrates, these you get rid of by doing water changes.
you want your ammonia and nitrites to read 0 ppm, and nitrates at around 20>ppm. (I'm a sps reef keeper, the thought of 20>ppm of nitrates gives the shivers lol)
I would add more hiding spots for your jd's to call their own, this is a key in keeping cichlids, everyone needs a little territory to defend.
to de-chlorinate, is to remove the chlorine and heavy metals in tap water, they are toxic to fish. they have a ton of products to do so
and lastly the sexing on the JDs is easy once they mature, the males will show more blue colored markings and have longer, pointed fins, the females speckles will look more lite green. it will be awhile before you can do this
lastly get a much larger tank as soon as possible, this guys get big, and need lots of territory