Poppy and Pepper sound like fun buns!
1. You may already know this, but if you are bonding rabbits they should both be spayed/neutered first. You didn’t mention if they are male or female so even if they are both the same sex and mating isn’t a problem, hormones can make some rabbits aggressive or territorial and it can lead to dangerous fights in the future. Fixed rabbits get along much better and may make the bonding process easier.
2. When Poppy trips or falls, is it mostly when she’s zooming about? Rabbits can be a little clumsy when they reach top speeds so if it only happens when she is excited then I think it is okay. She is still young, high energy, and probably getting used to her own capabilities.
3. If she is falling over all the time, regardless of speed or activity, you may want to take her to a vet to be sure her coordination/eyesight is okay. This is probably only necessary if it seems more neurological than just playful accidents.
4. If you are concerned for her safety, yes, maybe confine her to a smaller run so she can get used to the space. Since she is young she may not be used to her own speed and power, especially big feet, so a reduced space would contain her wild outbursts until you can expand it in small amounts over time as she gets older.
Good luck!