In reply to Fenda:
If you were fitted correctly by someone in store then I would stick with the Nepal Cubes. I bet your house is a toasty 17 degrees or so so don't worry about them being hot. Its when you are in the Scottish highlands walking/climbing on snow and ice all day that you will be glad you have them. You will need B3's for Scottish winter days climbing grade III and above as alasdair has said.
In terms of breaking them in there is not much wiggle room. If you are happy with the fit wearing them round the house the best advice would be to do a couple of long (10+ mile) walks in them before using them for climbing. Juso so you get a feel for moving in them.
If you are climbing big easy-ish alpine routes you could use a B2 boot. My friend did a few 4000ers in his old B2's but did say his feet were quite cold and had to keep moving. So B3's might be the way to go, especially if you plan on doing a few harder routes at a later date.
I have a pair of Nepal Extremes and have also tried the Cubes and will say they are so much lighter yet maintain most of the warmth of the Extremes. All in all if you are planning on doing more Scottish winter and summer alpine then you have the right tool for the job.