I was told they should be fine when i asked the same question a few months ago,and the big one was nearly double the size of the little one when i put the small one in and he doesnt bother him at all,the big one is 9" now and the small one is nearly 7 " now with my two endlis in the same tank which are 7" and 6" and they all seem very happy,hope this helps.
It's difficult to say, and is one of those things you just have to try, and be prepared in case it doesn't work out.
I had two juvies (6-7") in a 180g tank, and they fought over who got to 'perch' on the driftwood.
Rich, the owner of InfiniteAquatics, has a beautiful setup with I think 5 large ornates, big brutes that all look great. I think one was bullied, but the others got along fairly well from what he told me.
That's another variable type answer. That's one of the things I like about polypterus so much. As easy and straight forward as their care is, they each seem to have personality/quirks about them.
Anyways, the gills usually are gone by 4", but you can and will see some adults with small tiny stubs of their external gills left over.