I have a friend who uses Spirit Essence products. She said they worked better than the Bach Flower remedies.
I havent found that it makes much of a difference myself.
The only thing that is going to calm a feral kitten is the step to socialize them. They need to get use to your voice, touch, presence.. just like they show in the Urban Cat League videos. Feral or Domestic kittens.. they are all crazy wild guys.
I've used Rescue Remedy on dogs and it's works. They don't get all dopey but they do mellow a little. I think time and tons of attention is about the only way to get them to accept people. My momma cat's last litter were wild little dickens but several neighbors and I worked and worked very hard with those little guys and they finally came to accept us. Momma is another story tho, she's finally stopped hissing at me and will come up to me to be fed but I would not think of touching her yet.