Until they pair off, and while they are small, six in a 55g is fine.
If you get a pair or two (or three!!) that start breeding, they will become very territorial over their breeding area especially while they have eggs there - not only to the other angels, but to pretty much everyone else in the tank. With a 55g, there's enough room for the other fish to hang out elsewhere if you only have one breeding pair - if you get more than that, you'll need to rehome one of the pairs. But there's no guarantee that breeding pairs will form, so as long as you are prepared to rehome fish if needed, go ahead and have a few small angels in there now . . . .