I spent some more time this morning watching Pie and Drift interact in turnout. When I turned Drift out, Pie was grazing with Scout and Fritz. Drift drove him off, but Pie didn't go too far away - that didn't look too bad. That happened a few more times. If that'd been all that happened, things would have been OK. But next thing, Drift started leaving the group of geldings and aggressively running Pie at high speed down the fence line to the far corner - Pie has an unfortunate habit of sticking to the fence line nearest the barn and then allowing himself to be trapped in the corner (I hate pastures with right angle fence lines as they're dangerous, but's that what our barn has). Thankfully, once Drift had Pie pinned in the corner, Drift would only threaten to kick him rather than actually kick him - if he had kicked there wouldn't have been much Pie could have done about it trapped in the corner. Every time Pie would try to come down the fence line towards the gate, Drift would run him down again. This also meant that Pie wouldn't have been able to reach the water tank. By the time I intervened, Pie was standing in the corner, looking stressed and not grazing. I managed to grab Drift - who didn't want to be caught - and took him out of the pasture, and put him in a separate pasture. Pie went back to grazing with the other geldings. Pie's up to 4 1/2 hours of grazing, so he isn't out at night with the other geldings yet, but he should be soon as his feet seem to be coping well with the grazing.
For now, Drift will be having solo turnout - he's in sight of other horses and can even sniff noses with the mares under the electrified fence. I can't explain it to him, but if I could I would tell him that if he wants to act like a stallion, he can be treated like one, and at this point solo turnout is best. Not ideal from my point of view, but we can cope and Pie will be a lot happier.