I currently have seedlings, but will provide nutrients once the plants are ready. I’m going to mix my nutrients a gallon at a time. Considering that I only have 2 plants, I doubt I’m going to use the entire gallon of nutrients in one feeding. Can I store what’s left? If so, how long before it goes bad?

You'll get different answers from different people on this one. It's best to ask the manufacturer because they know their nutrients best.