I've been making a lot of fruit beers lately, and actually have done one not too different from this in the past. It was a a sour mash raspberry. Here's a link with some information on it:
When I did my sour mash, it accounted for about half of the fermentables. I'm not sure if a 1lb sour mash with 6 lbs of DME is going to give you much perceptible sourness. Also, you should boil the soured wort before combining it back with the rest to kill the lacto - otherwise it'll go nuts on the rest of your wort.
Be careful when heating the peaches, because if you get them too hot you'll set the pectin, leaving a heavy haze in your beer.
I have found that racking onto fruit in the secondary doesn't impart of lot of fruit flavor because the yeast loves the fructose and will eat it all. What I've been doing lately is making a beer (usually a wheat) then stabilizing with K-Sorbate and "dry hopping" with some dried fruit (like apricot or dried mango) in a mesh bag in my keg. IT helps keep the fruit flavors intact a bit better.