There is one option that I don't care for because it is not stable in it's effect, which is to put something of calcareous material in your tank. The acidic water will slowly leach the minerals out thus raising your gh and ph. But this process is continuous to some degree and depending on how much you put in can push things further in the other direction than you may desire. Limestone, dead coral, shells and such would do for this.
You might experiment with a small pieces in your filter if there is space for such in there. And by experiment I mean try various amounts until you get the rise in ph and gh you would like without going to high before next water change.
You are right about guppies being normally found in harder water, but they are able to adjust to a range of conditions. They might not grow as big or vigorously in acidic water due to less available nutrients for bone growth that exist in harder water. Dwarf cichlids, many tetras, some of the gouramis, other betta species than the fancy splendens... those are just what I can think of off the top of my head on soft water species. And even those often only need the softest water values for spawning, and the rest of the time they do just fine with slightly harder more stable water.
Just be aware that stocking levels must be less because of the reduced minerals and lack of stability that goes with that.