I wouldn't give mobility drug with no reason. All drugs have possible side effects and, the more you take a drug the less efficient it is, so I'd rather keep something as potentially life-saving as mobility drugs for a time when they are really needed.
Like Watermelons I've never heard of them being prescribed automatically after a spay (and Watermelons and I are not from the same country ^^). Generally, pain meds are given to avoid anorexia and work very well. Sometimes some antibiotics to avoid a possible infection.
My does never stopped eating for more than 12 hours post spay.
To answer your question, probiotics are completely useless when given orally. I've researched the topic extensively when I thought about giving some to my current doe. Studies show they are destroyed when they reach the rabbit stomach by the acid it contains. It's apparently a defense mecanism against foreign bacteries. The studies showed that the only efficient probiotic is actually cecotrophes, considering that it passes through the stomach intact.
Honestly, discuss it with your vet, but if I were you I would give Metacam for 5-7 days post spay, keeping an eye on the litter box and the hay intake, and that's it.