Hmm.....Well, with that much perlite, you shouldn't have to worry about drowning the roots, even if at 12 hour watering intervals. Is this setup going to be used for the entire grow? IDK, feeding once every 7 days, isn't that extreme. I rarely feed mine more than every 5 days, and they do just fine. Just give then a tad more than normal, like 20%. Then if you see any deficiencies starting, make next week's dose a little stronger. Better yet, leave the fert mix lower, and get some kind of spray system that would foliar feed them once or twice a week. The plants will only absorb what they need, so no risk of overfert. Hell, you already spent $7K, so I figure some type of sprinkler system might be doable?lol
As for the PH...IDK, but I'd try adding some lime on top of a couple, experimentally, and see how that works out. That's just an idea though, so don't hold me responsible if it wreaks havoc, because I've only heard of growers using it to keep PH up, not to lower it. IDK, but it's supposed to help keep your PH in the neutral range, so it's possible that it'd drop the 8.0 down closer to the 7.0 range. Hopefully, somebody has more input on this, because that's just an idea. I've never had to deal with high PH, only low.