>So you would take a frame of brood, honey, and pollen from the hive I've just started from a package and use it to populate my nuc? Would that possibly put the starting hive at a dangerous disadvantage for numbers?
Probably not before it has two deeps full of drawn comb and a good population of bees.
>I'm still kinda confused on this as well. How do you maintain a nuc at that size?
You can maintain it that small by stealing frames of brood for other hives and giving them empty combs to draw. Or you can move it into a full size box and let it grow.
> If my hives are going well and don't need anything, what do I do with a growing nuc?
Sooner or later you hives will need something. Meanwhile you get to experiment with raising a queen, nursing a small nuc and you have resources you can steal for other hives without worrying about setting back one of your production hives. The nuc is expendable. You can combine them to save a queenless hive or just steal open brood for a queenless hive. You have more options and more resources.http://bushfarms.com/beesnucs.htm