Prednisone with laxatives?

You can take laxatives with prednisone however some may be more preferable than others.
In general, laxatives should only be used on a short-term basis at the recommended dosages. If you are taking large doses of Prednisone then there is a risk with the more potent laxatives such as dulcolax, to develop hypokalemia, an electrolyte inbalance. Signs and symptoms of hypokalemia include fatigue, muscle pain and muscle weakness.
In your particular case exercise is important and try to increase the amount fiber in your diet. You should consider the use of safer bulk-forming laxatives such as Metamucil.
Talk to your pharmacist for more advice.