Over running the SFR is not a good idea and you should get the correct sized softener and program it for the best salt efficiency it can give you. That's because you should want to get it done right the first time and gt on with life.
An metered 8 day regeneration is fairly efficient and if you program at 60 gals/day/person there won't be much capacity left in 8 days to worry about. To change to a larger tank measn you have to change parts in the control valve also. And the plumbing and buy more resin. It's a bad idea.
The softener drain water needs to go to the septic/sewer system, it is not good for a pool and will mess up the water chemistry due to the hardness possibly but more likely the salt.