Regarding the Level I Tango Intensives, does the letter signify the difficulty of the class? For instance, is Series E harder than Series A?
No. Each series has its own technique, completely independent of each other. The classes are not progressive, so you can start taking the Level I Tango Intensives
from any session. The only prerequisite is that you have taken the Absolute Beginner Tango Intensive at least once.