You can most definitely have both, as unconventional as it may be, doesn't mean it's wrong! You'll have to define the purpose of the canister of course. Perhaps use it as a method of carbon filtration or perhaps as an intermediary until you receive more robust items to filter with. It doesn't hurt, but may also not be needed.
The purpose of a sump is to hide equipment from the display. Down the line, you may find yourself not having a purpose of the canister. So long as you have a proper means of keeping your tank balanced and tuned, then your need for the canister is non-essential and you could remove it then.
Edit: Oh Bevo got me by like 10 seconds!!!! Looks like we're on the same page. Cheers all around!