The Teensy lib might be used for more complex scenarios, that means when the "default" Arduino functions like analogRead() and analogReadResolution() are not enough to achieve what you want. Shortly resumed: It depends on your project.
Before you start coding, you should setup your algorithm, be it in mind or on paper, and define the specifications, first: How many signals will I have to read, at which intervals, regular depending on the signal bandwidth or asynchronous, and at which resolution? Do I (depending on the environment) need filtering functions like averaging? How do I want to process the readings afterwards, can it be done in real time on the fly, or do I have to buffer a series of readings before processing these in a batch?
If you have answered these questions, that means if you know exactly what you need, the second step is to look for the appropriate tools: Can that be done with the Arduino functions, then do it with the Arduino functions. Can't it be done with the Arduino functions, check if the ADC lib can do it. Can't it be done with the ADC lib, write your own ADC engine.
That's what we call systematic problem analysis and project engineering.