Can scallions be used in place of leeks

It depends. If you are making a stuffing or something sturdy like that, onions might be a better choice, as the scallions would just disappear, flavor-wise. QueenSashy is right---scallions are more delicate, which is why they are better for finishing a dish or in a dish that doesn't have much cooking. Leeks, on the other hand, need some cooking.

... and if you are going to cook them, you may want to reduce cooking time, because scallions are much softer.

Mostly yes, unless leeks are the featured ingredient of the recipe. You may need to double the amount of scallions to get the equivalentof the leeks