It could work, but off the bat I wouldn’t recommend it — it would almost be like having a bunch of chimps in a lion’s cage, even with lots of trees and vegetation.
You could try it out and see if the Celestials are in a constant state of fear… but I wouldn’t even bother… why subject them to this kind of possible stress? Mine have their own (densely planted) tank and are very happy… with fry co-existing with them.
If you really want to do it, I recommend a much larger tank, at least a 10-gallon. Read this article : http://www.fishforums.net/index.php?showtopic=96682 (white cloud minnows are close relatives of Celestials in my opinion).
Any other thoughts from other members?