I mean, you could do six if you wanted to do smaller things in a 55, such as appisto species (I know nothing about them though other than they get to be 3" or so) or Rams of some sort.
Or angels/discus....but those might not be what you had in mind as cichlids for the time being.
If you wanted cichlids that got up to 5" I'd only do 4, six might be pushing it (though tbh I'm not sure), but as for plecos, there are a few that would work in a 55 with plants. I know that any of the numerous bristlenose pleco varieties and species would work, outside of that though I am a bit unsure (though I'm also looking for a pleco so we have a similar issue). One thing I'd say though, is that if doing any sort of pleco with plants, especially more delicate plants, is that you should make sure the plants are established well first before adding the pleco....otherwise that might be a good deal of replanting once things get accidentally uprooted.