first...not trying to be harsh...a 30 gallon and a 20 gallon differ in volume by 50%...no matter how you do the math. So if they say a minimum of 30 gallons...doesn't mean you can't do whatever you want to...add a shark...20 gallons is perfect for a mini reef and a great start...but it is what it is.
Dwarf angles all play off the same game plan...they sing from the same hymn book. Yes, there is variance...but in general they are omnivores that will eat about anything that looks tasty. In the wild, they range a pretty large area, unlike grammas and firefish who stay put. So a small space is kind of cruel for them.
I am just trying to be helpful...just my opinion along with everyone else's. Weigh that with what your going to do anyway and enjoy.