Whats the ideal pH? It depends on quite a few factors. What kind of fish do you want to keep? Will they be locally bred? Or wild caught? What are the water conditions at your LFS? etc.
I've kept "soft water" fish in my tanks quite successfully. I have a few angels, neons, blackskirts, and various other tetras in my tanks for quite awhile. My blackskirts even attempted to breed one time (though it was unsuccessful... they did it in a community tank =\). My water is extemely hard... I haven't actively measured my GH, but I know its on the high end. The calcium deposits on my facuets, showerheads, etc. speak for themselves.
Just a note, it is much easier to raise the hardness of your water than to lower it. In the case of a Lake Tang setup, adjusting your tank to accomodate the fish would be much easier than to lower your water's hardness to accomodate wild caught soft water fish.
HTH
-Richer