All cycled tanks should have at least one water change per week. It's the only way to get rid of dissolved organic matter like hormones that we can't test for. Depending on your stocking level, you should do between 25% and 50% weekly water change.
While you are doing a seeded cycle, you'll have to test the water daily for nitrite and ammonia to determine how much water you need to change. Just remember, doing a 50% water change reduces the ammonia and nitrite by 50%. When I did a seeded cycle, I ended up changing 50% of the water every 3 days, but I also seeded from a tank of equal size and had fast-growing live plants.
Prime can be used in doses small enough to treat a gallon. When I refill my betta tanks, I use a one gallon bucket and dose it with 0.10 mL of prime. I use a diabetes syringe to get the dosing right. I don't think I could do it with just the cap.