The small bottle has the identical opening as the large bottles! It therefore fits.
Probably don't choose the maximum amount of carbonic acid for it, as it is only 1/2 litre of water.
On the manufacturer's page it says : ✔ Compatible with all SodaStream devices (except Crystal and Penguin)
Unfortunately, I had to find out after the purchase that it probably doesn't work with our Crystal. How stupid... I thought there were standards... -.-