If you're worried about the exact amount of co2 you'll need in your tank for the plants to thrive, I recommend getting a co2 drop checker. It basically tells you when you're using the right amount of co2, because a drop checker uses a solution that changes color to inform you, blue=too low amount of co2, green=perfect amount of co2 in the water, and yellow=means too much co2.
I use them in all of my tanks, I love the little things lol, because I always know if I'm using the right amount of co2.
Even if you add more plants it'll help you with what's the right amount of co2 in the tank, because the more plants you have in a tank the more co2 you'll need to compensate for the change.
I would go with a 10lb co2 cylinder, simply because the bigger the cylinder, the longer it will last. So say a 5lb cylinder lasts you 6 months, that means a 10lb cylinder will last for a year give or take, and means not having to get your cylinder refilled as often.
15g Fluval Flex - Will be a planted shrimp tank