Congrats on getting your doctor to certify that you have a qualifying condition. You should receive your card soon. I would suggest you discuss how using medical cannabis will impact your probation. If a condition of your probation is that you cannot drink or partake in drugs don't do it without permission. Although the PO has to uphold state law, he/she may not agree with the program and may turn you into the feds. Just be careful. There isn't a product out there that can detoxify you for a urinalysis with no notice.
Good luck and welcome to the program!