Once you add the CYA, you won't need to add any more except to "top it off" because of splash out and backwash losses, etc. 25lbs will probably last you several years, unless you plan on draining and filling your pool up every season, etc. You can always buy a couple of stabilized chlorine tabs to top off your CYA, as well. I'd say go with the 7lbs, or maybe if you can find a 10 (I got a 10lb bucket at Leslie's back in the day). It'll always be available, as I believe it's a product from industrial waste.
As far as how much you need, you can do the calculation this way. You said you need 7lbs for your pool. So, if you have 3 month swimming season, let's say you backwash weekly ~ 200 gallons. And lose another ~50 gallons to splashout, so 250 gallons per week for 12 weeks = 3000 gallons. The amount of CYA you'll need is 3000/15000 * 7 ~ 1.5 lbs per year. A 25 lb supply would last you about 10-12 years, if you don't drain and fill!
Good luck!
Take care,
Mo