You've got the general idea.

If you're soloing or with friends, you can do whatever you like, and go outside the box. Several people in my FC run all-SMN dungeons. At low levels, lots of people form full parties and use ACN as the healer (or tank, with yellow carby) for dungeons.

But with random people, you'll be expected to fulfill the role you're in. Scholar is a healer, ACN and SMN are DPS. They can't queue outside those roles. If you show up to a dungeon as a healer and try to DPS instead, people are not going to be pleased.

They're also not going to be pleased to see an ACN instead of a SMN or SCH much past 30. ACN just isn't as strong a DPS as SMN, or as strong a healer as SCH.