You are in need of an odd piece of equipment. You fall into the low usage, low hardness, high flow category. Considering the pipe size, # of baths, etc. I would recommend a 2.5 Cu. Ft 7000 (13x54 tank) for both the softener and GAC system. Install the GAC first, ahead of the softener. Set your salt to a low setting and put the override at 30 days, and you will do just fine. The GAC system can backwash every 14-30 days depending on sediment in your water. With this design, it is very unlikely you will have any noticable flow reduction. Be sure to order the 7000 iwth the plumbing connectors that match your plumbing size.