The supplements you describe are sometimes used in rabbits, with variable success. There is no strict dose, but half a capsule would do no harm. It can't hurt to try.
But before you give your bun any medication, it's important to have your bunny seen by a rabbit-savvy veterinarian who may be able to diagnose a problem you don't suspect. Stiff walking, for example, can be due to a parasite (E. cuniculi) that can also damage the kidneys and nervous system. You don't want to treat for arthritis if a problem is really something else.
I've lived with companion rabbits for more than 35 years, and consider them members of my family. I can answer any questions about the biology and health of rabbits, from the commonplace to the unusual. But please note:
RULE #1: THE INTERNET IS NOT THE PLACE TO SEEK HELP IN AN EMERGENCY.
If your rabbits is LETHARGIC
If your rabbit is NOT EATING
If your rabbit is PHYSICALLY INJURED (including broken bones)
I have been rescuing and rehabilitating domestic and wild rabbits for about 30 years.
I have written articles for many rabbit rescue publications, as well as for the veterinary journal, Exotic DVM.
I own EtherBun, the internet's largest listserve dedicated to health, care, and behavior of domestic rabbits.
Organizations Houserabbit Adoption, Rescue, and Education, Inc. (H.A.R.E., Inc.) president
National House Rabbit Society (Board member)
Publications Exotic DVM
Warren Peace (Journal of the House Rabbit Society of Miami)
Various newsletters of the House Rabbit Society, nationwide
Education/Credentials Ph.D - Biology
B.S. - Biology
B.A. - English
Awards and Honors Lightspan Academic Excellence Award for web site on rabbit health and biology