Some things about calcium that will help you with your question.
You probably know that vitamin D3 is used to help metabolize calcium. It allows for it to pass through the intestinal wall, into the circulatory system. Any excess dietary calcium is passed in the feces and not absorbed.
Sooo, if the animal has a natural amount of D3, it won't absorb too much calcium and you will not be overdosing it (withen reason of course). The animal should be recieving it's d3 either by producing it's own, and/or from it's diet (moreso in the nocturnal animals).
However, if to much D3 is present, the animal cannot regulate it's calcium absorption, and you can get a calcium overdose. Thus, I recommend a calcium supplement that does NOT contain D3. This vitamin should be occuring naturally in a varied, wholesome diet.
Symptoms of calcium overdose (through dietary means) would include swollen joints, thickened legs, and perhaps signs of arthritis and/or lack of urine production, and abdominal pain. If your pet becomes sick, a vet will discuss all possibilities with you 
Ian