vinegar... ._.
wont vinegar kill your fish?
anyway what will you do when you have to change the water? new water change water will be coming in at 7.6pH...
that pH is fine for alkaline fish, i dont get what your problem is? you think that the tank has to be at 7pH in order to be right? I'd just work with w/e the tap water gives you. you can keep rainbowfish in that pH.
brown stuff that grows on glass is diatoms.
I dont think I have seen any of my tanks get cloudy outside of bothering sand into the water column and back when i stupidly added a pH buffer to my tank, that got my tank very cloudy for at least a day x_x
If I remember right, during cycling if the tank gets cloudy that means a spike in ammonia-consuming bacteria.
anyway good luck with the tank, we are all curious to see which fish you will go with.