I can't really ad an more good advise, you've gotten plenty already. Christine, you handled Mick and you were able to make huge strides in his problem areas, so don't doubt for a minute that you won't have Sep straightened out with some time.
As far as the vet staff goes, I wouldn't really worry too much about what they think. It's only going to keep you feeling anxious and honestly your worries about what they think are probably far worse than reality will turn out to be. Most clinics would welcome an owner that acknowledges their dog's less than desireable behaviors and attemtpts to keep the dog under control and avoid chaos. They may be slightly concerned about him until they get a chance to meet him but if he poses no threat to them personally, I'll bet they won't care if he has issues with other animals.