I wouldn't call your water "soft" compared to where I've lived, but I haven't kept guppies in it either. I "think" they'd be fine like Joel (FF) said. I'd say go for them!
It's MUCH easier to keep fish in your tap water parameters than to try & hit the "perfect" parameters. Really, my husband is a chemist & when, many years ago, we would read the "ideal, wild fish" pH or hardness, he could get it there, lol. Needless to say, our early fish suffered for the roller coaster ride we put them through. Don't do it!! Either guppies will be fine (likely) or you should find fish that can live in your water.