What dosage of echinacea can be given to a 8-10 lb. cat?

Optional Information:
Type of Animal: domestic
Gender: female
Age: 12 or 13

Already Tried:
My cat has had problems for about a year with recurring fever and lethargy. She has an undiagnosed problem that began as a "neurological problem" (a quote from the first vet who checked her), whereby she would wobble and then fall over when she tried to walk. Her eyes were a strange burgundy color behind the natural color of her eyes., and she had relapsed every two or three months. She probably has a viral infection was suggested recently. She has been given Amoxicillin (100 mg. twice daily) with prednisone (5 mg. once daily) and, most recently, Baytril (1/2 22.7 mg. tablet twice daily for 5 days). I read that echinacea can help a cat's immune system, but I didn't want to just medicate her myself. Thanks.