a kibble with glycosamine in it, is not enough to support the joint structure. seniors, or any dog that has issues needs a theraputic dosage. 12-1800 miligrams a day. usually the joint supplements will come in 500 mils per pill. they need at least two or three pills a day.
if you are going to start this you should also add other supplements that work together with the joint supplement, like ester c, fish oils, hyaluronic acid, etc.
some dogs might be sensative to condroitin (gasy in the stomach) or certain sea based products, its trial to see which works best. if a dog has reactions to these, or even if they don't another great joint alternative is chicken collagen, which is absorbed better by the body since its more like the natural collagen the body makes.
i have never seen a dog OD, on a joint supplement, as long as you stick with regular recommended doses. the biggest thing is toleration as i explained above.