canister/sump

Can't say, as the whole circuit needs to be described before a definitive answer can be given.

Flow wise it probably makes more sense to filter the water after it's gone thru the sump. Not essentially competing with the sump.

If there is a pump after the canister or in the canister and the outlet goes over the the tank top and can in no way establish a reverse sipon on a power failure there is no reason you can't run them both from the overflow box.
 
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