A mild should be fine. A 1.035 mild will end up at 4.5ish ABV. Just keep the IBU's low on the beer you want to use for your sour so you don't inhibit the lacto.No lacto in this AFAIK. The 1.035= 4.5'ish you mentioned....oddly enough in BeerSmith, I am planning a 1.036 OG Mild, but this is showing 3.4% ABV
In the past I've done a split saison/sour batch, where I made one main all-grain batch that was as large as my equipment allowed. The two recipes weren't exactly the same, so I topped off the sour with some concentrated extract wort to boost the OG and diluted the saison with lower-gravity hopped wort to boost the IBU's. So if your recipe is close but not exactly where you want it, you can always top off with a little extract wort to hit the exact numbers you are looking for.
That is right for the Mild, but the sour will finish close to 1.000 and end up at a higher ABV than a typical Sacc ferment. A sour would typically end up with 1-2% extra ABV than a non-wild beer at the same OG because of the extra attenuation.
I haven't used JP dregs yet, so I'm not sure what the bugs are. But if the beer you are pitching from is actually sour (as opposed to just Brett/funk), then there is a good chance that there is some lactobacillus in the culture.