What an arrogant so and so! I would first and foremost find a new vet immediately. Apparently this vet has "never seen a reaction" to convenia used in cats because she denies all reports of those reactions.
Here is some good information on probiotics:
You don't have to be a raw feeder, or have an IBD cat to benefit from this website. Probiotics for Cats - Why and Which Ones - Raw Feeding for IBD Cats
Generally speaking you could start with Proviable DC, a probiotic made for animals, but you might want to take some time to read the link. https://smile.amazon.com/Proviable-D...ats+-+80+count
Another easy choice would be Natural Factors Double Strength. https://smile.amazon.com/Natural-Fac...ors+probiotics
I use both of these brands for my cats. Two take the natural Factors, and my IBD girl takes the proviable DC.
I have a friend whose cat eats the Fancy Feast classics and her little girl was having loose stools and diarrhea. I sent her a bottle of the Natural Factors and it cleared things up for her cat, and she's kept her on it permanently.
Start with half a capsule a day, work up to a whole capsule half in a morning meal and half in an evening meal.