If it's going to be external, then I don't see why you need a tiny pump. You can hide your canister anywhere, and overfiltration is better than underfiltration. You might as well go with a bigger canister and bigger pump. Flow rate is another matter, you can adjust that by packing your canister with more media, trimming the impeller down, or with an inline ball valve.
Keep in mind that with centrifugal pumps, generally the low flow ones are the low pressure ones. Also keep in mind that the media is going to cut down on your flow rate significantly. I built a canister using a Rio 200, for instance, and while that pump is rated at 138 GPH at zero head, I was getting 30-40 GPH depending on how recently I'd cleaned it.