Can I use a 10i filter in a 10 gallon saltwater aquarium?
Not a great idea, it'll become a detritus trap. Not a huge issue in freshwater but in a marine setup you'll get a lot of nitrates. 5-10lbs good live rock and enough flow to overturn at least 100 gph is what you need. I'm all for nano marine, but do be sure to stock and maintain properly.
Any internal filter will work to provide water flow, but as @bookshelf says you need to avoid detritus buildup. However, that one has too low a flow rate, the 40i would be better. Note: I'd look at other brands, including hang on back filters, whisper is not a company I ever recommend.