The problem with the syrups is that they have fermentable sugars in them, so you can't really just dose a commercial beer or some of your beer right before bottling to get a true picture of how it will taste in the end. It will dry out once the yeast eat the sugar from the syrup. I'd let your primary finish up, then rack some to a secondary and add the syrup there. If you can split a batch into a couple of mini batches and dose it with different amounts of syrup then that would be even better.
Flavor wise, the best bet probably be to add them at bottling time, but if you do that you'll need to decrease your priming sugar to account for the sugar in the syrup.