@ saskja: I'm not aware that tours only left after they'd gathered enough people (that's what you mean, right?), but I wouldn't dare to tell how common that is. Do you remember what company you used?
@ stewart: note saskja's heads up re the canal tours. For more information about the Jordaan area, see one of my posts above, where I included a link to more information.
As for the weather, I'm afraid no-one would bet how it would be when you're here. The past few days it's been close to zero degrees where I live (35km south of Amsterdam), with at times a wind chill making it feel even colder, this morning it's grey and foggy, and I'm sure it's chilly too. The coming days the wind will switch from N/NE to S/SW and that will bring lower temperatures (7-10°C) and a higher chance of rain, and according to the forecast I saw on the news this morning temperatures will drop below zero again during the night (and early morning) from next week.
A light sweater and jacket would definitely not be enough for me in when the morning temperature is around zero and the wind makes it feel even colder - I wouldn't go outside without my winter coat and a thick sweater over at least a t-shirt and possibly also a long-sleeved shirt. Jeans are okay, and I've never worn thermal underwear. You may want to protect your eyes, ears and hands if the forecast shows it will get really cold and windy.