Thank you for all of your help! The bun that needs it the most is my harlequin mix. She has always had issues with molting. I feed her rolled oats to help, which it does, but combing her with my human comb just doesn't quite do the trick and I didn't want to get a comb/brush that would do more harm than good.
I will definitely be investing in a rubber brush. It sounds like it might be worth a try!