there is usually very little material coming from the ceiling speakers... most movies use them for creating ambiance with subtle (often random) sounds so... pretty much any size speaker will do just fine for ceiling duties.
now, you have also movies like "unbroken" the first 5-10 minutes on the bomber when the ceiling speakers are running hard, but those movies are few and far apart and if you dont listen at very high levels the small speakers should more than be sufficient.
so, i say go for it.