well yeah but even though it's just barely over you'd have to change your water a lot even if it was cycled! like 50% twice or three times a week. =/ If you're up for all that maitenence cool. =] I just wouldn't be up to it. (I'm REALLY lazy. =P)
I think if you want guppies you should dedicate a tank to just them! =] That way there'd be no threat of fin loss due to nipping, and then you could have many more! They come in such a wide variety of colors too so it'd be a really diverse tank! =]
Also I might be wrong but don't they prefer different water parameters than bettas?