As I added a few bosses to my old Peugot frame, I noticed that it is easy to overheat the bosses when brazing, since they have little mass. After overheating one of them (red hot), it became obvious not to do that to the others. I was just wondering what the problem is in getting them really red hot. I hope they don't crack or shatter, and have my water bottle fall off the bike. It's an old frame, so no exotic steel, and it cooled naturally.

Assuming you got decent penetration with the filler before you burned the flux , don't sweat it, you'll be fine.

Keep the flame circling in a donut-shaped region just larger than the bosses. The frame and bosses should heat up to the same temperature.