Personally, I would feel bad having my female in anything smaller than the 3.5 gallons she's got... they really are very active and curious.
Her tank is heavily planted - the substrate, plants and wood all take up swimming space, too - so she's probably got about 3 gallons or less of that.. and as she's grown a little bit now, I am considering a larger tank. Which is to say, I think plants + decor + betta in 1.5 gallons is really too much. I'd recommend at least 3 gallons if not more, just from my experience.
If you used it as a shrimp tank, however, you could have maybe 20 shrimp in there - they're amazing fun to watch and I am way more attached to my 'betta snacks' than I thought I'd be (they are also getting their own tank soon). You can even get rare ones, if you have the $ and do the research. Blue, yellow, orange or even jet black. I would love to do this, if I had not a few tanks already and moving house soon..