I think it's tough because it's like a range. There's soil, where there's a lot of nutrients in the potting mix. Then there's coco, which is completely soil-less - it's just an empty growing medium and you add all the nutrients but the medium hold the roots. In hydro the roots just float in water, and you're adding the nutrients. It seems like the closer to a "soil grow" (the more nutrients contained in the soil) the more you treat it like soil. The more it seems like a "soil-less grow" (no nutrients in the potting mix) the more you treat it like coco/hydro.
I'm not sure if that helps, I'm not familiar with growing in peat. From what I understand Pro-Mix is a soilless mix, but I think it contains nutrients, so I think you could go either way with it.
I tried to look it up on the manufacturer website, and it seems to treat Pro-Mix as soilless. But they weren't really tutorials from Pro-Mix as much as random growing articles on their website by random people so I'm not sure it's necessarily Pro-Mix specific. I found three pages (
here,
here,
here) talking about pH. In the first one they say it depends on the plant, but the range for all the plants was 5.5-6.6. In the second page it says between 5.5-6.5. In the third page it's talking about using Pro-Mix in hydroponics, but they say 5.5-6 pH.
Based on those articles, I'd guess it's better to treat Pro-Mix like hydro with a pH in the 5.5-6.5 range, but just to be sure it can't hurt to
contact them! I bet they'll get back to you fast!