As far as I have gathered other than a "Sock" on a return, no mechanical filtration is need for a reef tank. I stoped using my canister filters and finaly got my nitrates to 0 or close to it. Befor with a mechanical filter they stayed at 20 or so.
I will on keep a canister filter and carbon ready if I need to run carbon for an emergancy.
It would depend on how you stock the tank and your feeding habits. If it's not over-stocked and there isn't a whole lot of food waste it should be okay. but why not just buy the larger canister? The price difference between the sizes isn't that big from what i can see.