Yes loans levels can very. I try not to ever have more than a handful. If you can keep a war going even with a tiny profit that is ideal naturally. I will go deep into the red if it is a matter of survival.
Enlarging your realm means making cores on the land you conquer. It costs Admin power. Expanding fast means spending lots of admin points. This will slow down admin tech, and delay the idea groups that you get at certain admin tech levels.Should I sell my Morocco provinces to weak Aragon instead of letting them fall to Moroccan rebels? I can conquer them later from Aragon.
You might be thinking of Personal Union junior partners—vassals can be integrated after 10 years. Otherwise, good explanation of vassal feeding.A small country can be made a vassal. Vassals can be held 50 years, then integrated with diplo power if relations are above 190. Any vassal core will become your core. This allows you to add more provinces with less admin power cost.