I’ve probably not a percentage of the experience of Sabrina, but I’ve used both the indoor and outdoor method in the last four years with all sorts of peony seeds and the outdoor method for me is much less hassle than the indoor method, with greater chance of succes. No need to check the progress of your seeds, no trouble acclimatising to the real seasons (provided you start the warm fase around the beginning of june) no worries about mold/fungus, no changing of irregular sprouting seedlings to other bags with risk of contamination etc. Species peonies must be happy outdoor germinators by nature, assuming there is not to much of a mismatch between natural environment and your garden climate. Perhaps special (intersectional?) crosses can use the extra care, but even my advanced lutea crosses seem to have no trouble sprouting in the outdoors method.
I would put the peregrina and tenuifolia outdoors now. Even if they germinate -which I doubt-, the first stage is only a hypocotyl, wich can withstand frost. The epicotyl won’t appear till next spring. You can of course keep them indoors, but I would not expect them to germinate without some more drastic temperature changes. What puzzles me is that your rockii have sprouted in the fase that you refer to as the cold fase. To my knowledge hypocotyls grow in the lingering warmth of the fall, and not in the cold fase. While epycotyls grow after the cold fase when things start to warm up (begin of spring). So what you see, might be seedlings that have only just started in the first “fall” period and need more time in this fase. Putting them outside will problably halt their germination without a sufficiently enlarged hypocotyl to live. I would put them back in warmth a little longer and transplant them as soon as their hypocotyl is 3 cm in an outdoor container. They will probably show their first leaves next spring (not this spring). So in both cases nature can’t be bypassed.
Pfff,, complicated. My message is much longer than planned, my apologies. Maybe the take home message is: start those wendelboi seeds in juni, whether outside or inside!