Ornates aren't very aggressive, so you can keep 2 together no problem. But, it also depends on the attitude of the ornates you get and your tank size.
Mean fish + small tank = BAD
From my experience, polys aren't very fast growers. I'd expect .5" a month, but once it hits 7", it may grow about an inch a month. Once it hits 14-20" it will slow down.