I keep a dwarf angel (coral beauty) in a 40g, and he is fat and happy. This is really going to be more Opinion based. If he gets stressed in your tank you can always return him. Remember they are hard to catch
I've had a Cherub in my 60g reef cube for about a year. I got it from someone who had it in their 60g cube for two years prior. It's been fine...no aggression with my Chalk Bass, Coris wrasse or lawnmower Benny and no nipping at corals. I would not go smaller. For dwarf Angels (not Pygmy) I would go 3-4' minimum.
For a cherub, 24g is fine. Each specimen is different as far as temperament. I have seen peaceful specimens in small tanks, aggressive ones in large tanks, and vice versa. But I have seen perfectly healthy ones kept in 20g.
can they live in a small tank? yes. I think the problem is the smaller the tank, the more aggressive they can become. aggression can cause stress in other fish which can lead to lots of problems.