This shows why I always insist on numbers. A GH of 4 to 8 dGH is not hard water, it is soft (the 4 dGH) to moderately hard (if 8 dGH). This is not hard enough for livebearers (or rift lake cichlids someone mentioned earlier), but will suit many of the "common" species very well.
The high KH of 20 dKH is something that will affect the pH, serving to buffer it and prevent fluctuations, so the pH in the aquarium will stay close to the tap water. But it is not too high (you would prefer it higher for livebearers again) so you are OK there. All told, pretty good water with respect to parameters.
So, to your initial question on the pleco and tankmates. As someone mentioned, make sure the pleco you acquire is not a "common" that gets way too big, but one of the smaller species that will remain 4-5 inches. Like a Bristlenose for example. As for tankmates, there are so many I don't want to suggest them. Have a look around and see what you might like, then ask us. Always, always research a species before you acquire it; some have more specific water requirements, or habitat needs (the aquascape has to provide these, like at least one if not more chunks of real aquarium wood for the plecostomus is essential), or numbers of the species if they are shoaling fish, or behaviour issues...there is a lot to combining species and having a healthy thriving aquarium. I prefer to advise on species rather than suggest some, at least until you get started.
Byron.