Hi badger. My dog was given convenia just last week for his skin irritation. We had a bad experience with our old vet just giving our dog meds and not telling us what they were, so our new vet always sits down and explains things to me prior. The first thing my vet told me about it was that they commonly use it to treat bladder infections in both cats and dogs, but it is also effective with skin conditions and irritations. My guess is that it's a safe treatment for your kitty. Of course there are risks with any drug, but she said it was one of the milder, common ones.
I'm sorry about your kitty, Gypsymagda