Two 5 frame nucs generally fit fair and square on a National box. Something to consider in the overall plan.
As said above, you can fill the space but can't, if it ain't there in the first place!
Set aginst all this, is a larger nuc will build up more qiuicky than a smaller one. So numbers in a while or a good strong nuc sooner.
I am going to go with 3 off, on 14 x12 frames this coming year - trouble is, I generally run warm way, so that may an added variable to contend with. We shall see. These will be in M*odern Beekeping jumbo Langstroth nucs (6 framers), padded out to accept National frames and divided into two - they have an entrance at each end, so little drifting into the other side, I hope. They are not the ideal situation for uniting to a national with newspaper, but I can live without that facility.
Regards,RAB