Yeh, the rating of SWCGs by pool volume varies a lot even within manufacturers, so you're not alone in the confusion. Going back to one of your PMs, I feel bad for saying a 15K SWCG is fine for 6,000 gallons, without asking more questions. I should have checked your thread and would have seen 7,000 gallons, but the bigger difference is that your equipment list here mentions Hayward, and that T-3 rating has always concerned me. I'm not bagging out Hayward, because I see the same thing for the Pentair IC15. Anyway, too much detail.
I think of it this way... 1.0 lb per day is good for 10,000 gallons under a wide range of conditions. If the T-3 is 0.53 lbs/day, that makes it a bit small for 7,000 gallons. Plus you have the benefit of longer cell life with the T-9 for not much more $.
The T-3 would work although it might be dictating your pump run time, or requiring chlorinating liquid additions on big swim days or hot spells, and it sounds like you're going to be keeping it pretty warm. The T-9 eliminates all or most of that, plus now knowing it's Hayward, there's no worry about not having enough settings. When I didn't know the brand, I had to consider having only 25%, 50%, 75% and 100% for options, which is troublesome with a way-oversized chlorinator. The Hayward has plenty of steps and will work fine. Don't be surprised if you're running at 10% or 20% a lot of the time!
If for any reason it doesn't work out, you can put in a T-3 when the T-9 wears out. It's just a software setting to change the cell size, and in the meantime you benefit from the small incremental cost to go from a T-3 to a T-9
If you can change stuff, it changes my response, cause I'm a 'what's done is done' responder. I still think you'll be happier with 3/4 HP or even 1 HP, and the reason is skimming. When you want to use the pool and for whatever reason the pool is covered in dead bugs or pollen, it's nice to skim the crud off quickly. The 1/2 HP will definitely do all the circulation and filtration you need. With a big chlorinator, you can run for relatively few hours 90% of the time, so I don't think total power consumption will be much different. When I was building the pool here, I came to a clear conclusion that 3/4 HP was lots for chemistry and killing algae, but I'm glad I have 1.2 HP for skimming, especially when it's also a bit windy. I also noticed the Hayward is a medium head pump, and a low head pump would be ideal, but anyway, I'm up way too late and going to bed! Hopefully the better pump experts will chime in or say you're good to go and I'm writing too much!
