the joy of homebrewing has a great appedix about extract sour mash brewing incorporating a sour mash and brett. i think that will be your best resource. i do all my batches with partial mash (i know, i know, 6 years brewing and still not AG). i like to think all my batches of sours have turned out just fine so no worries, you'll be okay
you should get some umalted wheat, flaked or torrified, and steep, then give a short boil to release tannins that will act as more food for the wild things.
ryane knows his sheeit, so if he says peaches won't work well he's probably right. that said, i would not let that deter you from using them. maybe throw them in with the mentioned apricots for an underlying flavor. there are guidelines for sours, but no "golden rules". when in doubt try it out