I did something like this, but a 20/20 for salt. I spent way too much money on plumbing for something that was an experiment. sch 80 bulkheads are expensive, but re-useable. you'd basically be using a MJ 900 for your return pump. now my 20 tank is plumbed in to my freshwater sump and acts as a nursery for my cichlids. on the 20 I currently use a danner mag 3 running through a BRS carbon reactor. it lowers the flow enough to not rage water everywhere.
I would rather buy a 20 gallon tank for 20$ and use the 10 as the sump. you're going to be dropping some money on plumbing anyways, and at least with the 20 it is more practical for future use. my cichlids will breed in the 20 long.