I don't know about the effects of glucosamine and potassium citrate in combination (best to check with your vet maybe), but the long-term use of glucosamine itself should not be a problem. Many brands of food have it added nowadays. The brand I was feeding until recently contained both glucosamine (625 mg/kg) and chondroitin (500 mg/kg) for joint health. The one I'm feeding now also has it in lower quantities (glucosamine 300mg/kg, chondroitin 100mg/kg). My neighbour gives her two older dogs a daily glucosamine supplement.
Beware of Mushers...They will suck you into their addiction!
If your dog thinks you're the best, don't seek a second opinion!