Rescue remedy has been used by a few of my clients for their pets. The report is inconsistent, some love the product, some don't think it is useful at all. Something to remember as a toxicological principle is that every substance in a high enough dose is dangerous, even something as common as water.
The ingredients of Rescue Remedy can cause gastrointestinal, respiratory and hepatic (liver)problems in prone individuals. If you use this preparation, then do so at published dose (should be on the bottle) and monitor your little guy over a two week period. Any unexplained problems, then stop the preparation.
It is important to take any concurrent medication into account when adding Rescue Remedy to the mix. I realize that it is popular to attempt what some call "holistic treatment", but one must be careful with any substance ingested. Know what the active ingredients are, what the tested strength is, what interactions can be problematic, any contraindications within that invidual, and the source of the product (contamination with heavy metals, pharmaceuticals, adulteration, etc. has been a problem in the past).
As for the seperation anxiety, your behaviourist should be able to give you guide lines. Usually I advocate a medication and training approach (both you and the little fellow are trained). I use a large array of meds, no two dogs are the same. Good luck.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124