Welcome to the forum. Do you have a KH test? You pH range is pretty wide, a pH of 7 is ten times more alkaline than a pH of 6. Also for most shrimp like cherries or bees, your low end temperature is their preferred high end. You'll want to back that heater down a bit. Shrimp are fragile for new tanks, most people recommend letting an aquarium run 60-90 days before introducing them. You might get away with adding them to a new tank, but there is going to be a much higher risk of losing them. Patience is a virtue in shrimp tanks. Finally, brown diatoms are pretty common in newly set up aquariums, I wouldn't necessarily blame the plant seller.
Nothing good happens fast in an ecosystem.