In a 7.5gal tank, you'd be able to house about a million Heterandria formosa without noticing them. I've found that larger Neo shrimp, which will be smaller than what you plan to keep, can also be a little aggressive toward H. formosa on occasion. Nothing too serious but it's enough that I'm always careful when keeping them with shrimp at first. I've had Amanos attack and eat them when they're mid-sized.
What kind of substrate, hardscape and plants are you planning? The more hidey holes the better. This is a terrific idea for a tank.
H. formosa are severely underrated fish but have been becoming one of my favorites the past couple years. One of the few fish that I don't hesitate to keep in smaller tanks. While they will develop quite a population in your tank, it won't be too insane like with Endlers or Guppies. But I do recommend having extra space for them in something like a planted vase, tub or small aquarium. Because you're going to fall in love with them and will want more.
