My first instinct would be to say what is good for one species is not necessarily good for another. But then again I guess it would depend on the vitamin. I don't have any reptical here to see the exact ingredients or percentages but I did have some Herpcare calcium. Looking at the ingredients and percentages it is in proportion with our bird calcium supplements. The main problem I can see is it gives dosages for iguanas and it may be hard to figure out how much a cockatiel would need. I have always found it best to just give birds (we don't breed but of course females sometimes still lay eggs) anyway we give them cuttlebone or "mineral" blocks and we mix eggs with shell into their diet. We have never had any problem with them being low on calcium this way but as I said since we don't breed the problem doesn't come up as often.
If you can't check ingredients to make sure of exact percentages and through a vet or the manufacturer figure an acurate dosage I would not use it. Some vitamin supplements have other forms of calcium which aren't as healthy or else have other vitamins mixed in which may give different species too much and not be healthy.
Stephanie