I think that I would be okay just leaving them but it depends on your soil and the shallot sets' viability.
If you wanted to hedge your bet, you might run down to the garden center and get a packet of shallot seed. I would also be comfortable sowing that in my garden location.
Although they are small plants, shallots are fairly tough. They can survive a winter after being set out in the fall (I don't do that) or planted in the spring. Harvest will be about the time as for garlic. Planted as seed, shallots will need the entire growing season, however.
We are each other's harvest; we are each other's business; we are each other's magnitude and bond. ~ Gwendolyn Brooks